Hello. This is my first guide so please be gentle, and now for the unique turtle, Rammus. Before we move on, I'd like to tell you some things.
Please do not down-vote, flame or do any other negative action just because the cheat sheet is incorrect in your opinion, or lacks anything. I will suggest you make a comment on this. I beg you not to down-vote or do anything else negatively before you have submitted a comment and I've answered you.
This guide is my way of playing Rammus. I'm in no way telling you exactly the best was of playing Rammus. Several sections of this guide will explain the different uses of items in different situations. This is all personal preference.
If there is something you dislike, please tell me so I can eventually correct it, or give you an answer to your question.
When voting, please tell me WHY you up/down-voted. I'd like constructive critism very much, as I'm not a professional guidemaker or player of any kind.
If you have a screenshot of a game you played you think is good, feel free to post.

Pros:
- An exceptional tank
- Easy to play
- Can save people's butts countless times
- Really hard to defeat with Defensive Ball Curl activated
- Is a turtle. Ok. How magnificent.
Cons:
- Easy to play yes, but hard to really master
- Tenacity is really annoying
- Very, very often banned in rankeds. At least in the lower ELOs for sure. Probably the most banned champion?
- More vulnerable than other tanks when Defensive Ball Curl is deactivated.

Marks
Any rune could fit here. Marks doesn't give a good defensive bonus, so you should use what you have. If you have 9x Greater Mark of Desolation, I'd suggest you use them. If not, anything else. These runes aren't too important for an tank.


Seals
Seals are probably the most important runes for Rammus. All I can say is Greater Seal of Resilience. Armor, Armor Armor. Rammus gains AD from Armor, and Seals' biggest boost is Armor. You can choose for yourself wheter you want flats or pr.level, but I'd prefer flats myself. This is due to the great help it gives you in the jungle: 1.41 x 9 = 12.69. It gives you a whopping 12 armor bonus in both lane and jungle, and this will be 30 with your cloth armor, along with 6 from masteries which will give you 36. Rammus' base armor is 21, which will reward you in having a sick 57 armor to start off with!!! As you can see on the graph in the previous section, the physical damage dealt to you will be reduced by around 36%!!! This bonus is SICK early game, where spells doesn't do too much damage. Wow. No doubt here.


Glyphs
As for glyphs, the best for Rammus to get would definitely be Greater Glyph of Warding or Greater Glyph of Shielding. This is really, really up to you, but you want to get MR for sure. Per level might be a good idea though, as these will kick in at level 7, and their effect will be bigger than the flats right there at level 7. But if you get flat MR at the beginning Rammus becomes more tanky for those early game kills. My personal preference is to get the flat MR and feed your carry the kills.


Quintessences
Greater Quintessence of Fortitude. Health quints are imo best, but it's up to you what you choose. Quints are good overall, and anything tanky would work. Greater Quintessence of Defence, Greater Quintessence of Warding etc. Nothing really much to say here for sure. It's all up to yourself. I just have to state that starting out with (26 x 3) = 78 more health will really help.

Summoner Spells

Alright, so this is the deal. I think Flash is a must. At least for myself. It has so incredibly many uses, and I just love it. Now, if you're going to lane, I think Exhaust would be your next best option. It is also good for playing late game.

Playing Rammus: Gameplay, Strategies & Teamfights

Lane


Grab your Dorans Ahield and set speed against your lane. Your best bet would be to lane against melee champions, preferably AD's. Your lanemate would preferably be a ranged dps.

Early Game just try to keep on last-hitting, and leave the harassing part to your lanemate. You might want to be sure that your lanemate gets a bunch of lasthits to get farmed for sure. Also when you see an opportunity, don't be afraid to try to roll in, taunt and activate your shield to grab a kill for your team. Make sure to mark the target and tell your lanemate first though. If you're going for the kill, make sure the target don't have flash. If it don't, pop your ulti; Tremors. It has really low cooldown and decent damage.
If you're pushing a tower, don't be afraid to pop your ulti either. It can help taking down the tower's health a lot faster, but make sure there aren't any enemy champions in range to get hit by your ultimate. If your tower gets pushed, taunt them with Puncturing Taunt, make sure you're in tower range, make them hit you once and run towards the tower to make the tower hit your target as long as possible. This is a great harass.

If you're solotop, play defensive and wait for the creeps to come to you. Rammus doesn't have a lot of damage output, but due to his passive it's a little early on; Spiked Shell.

Make them do the mistakes. If they overextend, make them pay for it by using Puncturing Taunt. When maxed, it can give your target 2-4 turret hits if used correctly - huge damage early on!

If you see a situation where there's like 4v4 mid, just pushing eachothers up and down without really doing anything but putting a small harass every now and then, look for opportunities, or create them yourself, to taunt one enemy (prefer melee) and drag them into the middle of your team. They should be done in a few seconds. Now the battle is 4v3 and you have a huge adventage. If you can manage to pull their carries, it would be awesome! You make those battles easy wins.

Late game is basically sticking with your team. Baron if possible, dragon... If there's a "pause" go to a lane and don't lose time. Time is exp, and exp is money. Money is items, items are necessary. But yes, sticking with your team and doing what you do.



Teamfights is where Rammus are incredibly awesome. For a whole THREE SECONDS, you can taunt the enemy's most damaging player, making them vulnerable to all attacks from your team. They will be down in no time at all. Remember to activate your Tremors, and of course Defensive Ball Curl whenever available. If you see there's a spell getting channeled etc, use Powerball and set speed towards the enemy to interrupt it. If it's a longer lasting fight, you can get to use Puncturing Taunt several times.
Remember the technique to pull out an enemy and into your team, this will REALLY REALLY make a difference!!! It can turn a fight from LOSING 5-4 in deaths, to 0-5 for you. Amazingly enough. If you can pull out their carry, they simply can't do ****. Well except if it's a Tryndamere. He will probably pull off his Undying Rage and get the hell back in panic. This means that he won't have his ultimate in a minute or something though. Care if he's really fed, he probably will try to kill your entire team instead of rushing back in panic ;)

Playing Rammus: Using Powerball

Powerball is a very unique and nice spell. It interrupts stuff. Great.

Different uses:
- Travelling A LOT faster
- Escaping by ^
- Saving peoples butt
- Interrupting channeled spells like Requiem, Nether Grasp etc.
- Detect where an Akali, Twitch, Evelynn or Shaco is hiding
- Initiate [Wait in bush for it to charge up for a gank, or just roll into someone]
- Chasing someone

Patch changes:
In January of 2012, Powerball got a small... 'tweak'. Powerball duration reduced to 7 seconds from 8. Powerball initial speed increased to 30% from 25%.
In my opinion, this is a buff. You can travel a little less with Powerball turned on, however, you start off with a greater speed. This means that it's easier to catch up with enemies, you have to wait one sec less in a bush when ganking (make sure you don't activate it too early now though, if you roll straight in), and it's easier to get away (thinking this way - you get off fast so they can't cc you - hopefully).
The only negative side imo is the fact that you travel for 1 second less. However some of this is paid back in the speed, but not all. One second won't cover those extra 25-30% you would have another second before. (The 25% to 30% in the one second if you compare them). If you'd imagine a graph, you would travel further in less time before this way, but not much. It might save you 0.5-0.75 seconds. It would be life saving in one out of 274 situations.

^This is basically my thoughts about this now at the moment, before using it (wrote this pre-patch). Thank you.
In-depth:
Quoted:
The lifeblood of our little friend. Powerball has a thousand uses. It does everything one can ask of an ability and more. Powerball (PB) is a small-aoe nuke/movement boost/short stun/slow that can hit invisible units. Plus its cooldown isn't long at all. I am about to try to enumerate all of the tricks I have developed and learned for Powerball use. I have broken it up as best I could. Some of this you may already have thought of, but you never know when you might learn something.

I. Map Movement
Powerball allows Rammus to fly across the map every 12 seconds. Do not walk around when you should be rolling. The movement speed bonus should be used often. Rely on your mana regen and time in lane to fill you back up.

1) Fountainball: Hit Q and wait 2 seconds in the fountain. This will let PB's speed charge up and allow you to regen all of your mana. Right before the final mana-filling tick, leave the fountain and head on out to cause mayhem. This gives you a nice free boost out to your lane, and gets you to the first inhibitor a whopping 6 seconds faster. It may not seem like much, but in LoL, time is definitely money.

2) Basic map movement: When there is a fight that you are eager to reach, it can be worth using PB to get there in time to make a difference. For instance, it takes about 2 PBs (one of them a fountain freebie) + the 12 second CD to get to the second tower of top or bottom lane. You should arrive with about 6 sec left before you can PB again. You will have lost about 100 mana and still have enough to use all of your abilities and still have a chunk of mana left over. This means that it's worth it to get there as fast as possible.

Oftentimes, people think that they will be too late and so it is not worth wasting the mana to miss the fight. WRONG! It is better to be down 100 mana than to miss being able to get in there in time to taunt an enemy and save your friend, and possibly get a kill. This is almost always true. If you have a chance to get to a battle by using PB and you aren't doing something more important to take it. That said, if you do not have enough mana to make a difference once you get there, forget it.

II. Initiation
You are the tank. Make sure to get right into the thick of things and draw fire. Give your allies a chance to unload their spells and attacks. Powerball is essential to this. Few champions can take a surprise 300 dmg stun/slow without flinching. One of the strongest assets of PB is its high-speed impact scariness. Even Alistar wants not to get rammed by a man-sized, spikey, rotating armordillo. Use that! The two ways you will be initiating is with surprising speed or scary mind-games. These tactics are more important in initiating ganks, because in team fights your focus is mainly to hit as many as possible and get into the thick of things.

1) Lightning Initiation: There are two tricks to what I call Lightning Initiation. These can be used both to start a gank or a initiate a big team fight.

i) The Face Flash: One tactic people never seem to expect is the Face Flash. This can be both a mind game and a speed tactic. Make sure you get a feel for PB's length first. Then follow these steps. Turn on PB. For maximum results, dance back and forth behind creep wave a bit, so that he thinks you hit Q by accident. Then as your speed is getting high, casually get closer to the the front of your creep line until you make your move. As you roll toward him at high speed wait until you are about to hit his line of creeps, and Flash right onto his face. If you can strike at a time where he's farther forward than he ought to be (trying to last hit) you can Flash onto his back side, knocking him a little ways forward. Now is when whoever you have waiting in the wings to gank can come on forward. Just before the PB slow wears off use your taunt as you both hit your prey.

(If you are Face Flashing to initiate a large-scale team fight, go straight into their squishiest champion without stopping for mind games. Against whole teams it's pretty useless to work so hard to get behind the enemy.)


***Make sure to Flash before you hit a bunch of creeps or you ruin the whole tactic. I've seen a bunch of Rammi charge forward to Face Flash and fail miserably by losing their PB on a creep just before they hit the button. It's funny... but not when it's you.***

ii) No Flash Lightning: This tactic is less effective but is doable without flash. You need to start in the brush. Many people expect Rammus' PB to accelerate at them starting slow, giving them time to run around creeps to cause you to mi****. In order to prevent this tactic, simply charge up your PB for a little while (about 4 sec) in brush. You don't need to move for the speed to increase. After you hit about 500 MS or so, come around behind the unwary enemy and use the same PB-from-behind tactic if possible. Once again, your disconcerted prey will feel the pain as you and and your teammates rip him to shreds.

(This works for large fights as well, it is always more effective to come in with surprising speed.)

2) Mind Game Initiation: The most effective way to use Rammus in a gank is to come up from behind. However, you are most formidable while in PB roll. Don't waste any of the time you have in your PB. Instead of coming from behind and slamming into the enemy, PB in and play some good ol' basketball defense. Don't foul her, but mirror her movements and stop her path to the hoop. She will do his best to avoid you, but if you continue to block her path out he will usually do something stupid to avoid you (like running back toward your friends). By being a large, spinning escape route blocker you effectively increase the time the enemy is forced to stay where you want her. As soon as PB has served this purpose or is about to run out, then make sure to ram her from behind and knock/slow her into your team.


III. Escape
I touched on this during Summoner Spells, but PB's escape usage needs a stretch of free space. The basic goal of this section is to teach you how to make yourself an open runway, and when to save PB until you've made that runway for yourself. Remember that once your PB is down you are pretty much trapped for 12 seconds (AKA dead).

1) Group Flash Escape: When you have Flash up, this escape move isn't too difficult. Use DBC to hold off fire until you get to a wall to Flash over. Cancel DBC just after you Flash. As soon as you get over the wall, start your PB and zoom off in any direction other than the one with your pursuers. Naturally try to avoid their creep waves/towers. If you can get away anywhere unseen, PB can take you far enough that they are unlikely to find you before your blue pill gets you safely home. In this situation do NOT use PB before you Flash. It's likely that one of your enemies will take the impact and you will be stuck for 12 seconds.

2) Escape without Flash: This is much more difficult. For this you can't be trapped in the middle of a large group unless you have some teammates helping you. You have to try to run a little ways out while DBC is up, and as soon as the coast is clear, PB out of there.

The most dangerous part is during the opening stages of PB, while it is slow enough to be interrupted by an enemy trying to run into you and block it. To prevent this, use your juke power. As you try to make it out, run into some brush before you PB. Don't immediately use it. Stay at the very edge of the brush. As soon as the enemy enters the brush, or even right before, run out of the brush and use PB. Most enemies will not be able to react in time to catch you, meaning you can escape freely.

IV. Chasing
I've seen many young Rammi think that PB makes them powerful tower-diving chasers. They are only partly right. You can't chase too far into dangerous territory with PB alone. If you get into a hostile situation against a tower and/or enemies without PB up to escape, you can be in serious trouble. Only chase when you can cut off an enemy far from their tower, they are very low HP and there is no chance of other enemies in the vicinity, or you have allies with you. Don't chase too far from your allies either. Usually you end up ramming an enemy alone and with low mana, and you end up either dead or severely injured. So what should you do?

1) Be the blockade: Because your PB has the potential to get to the enemy with your allies trailing far behind, don't just run straight after the enemy. Let your slow allies do that. Go the long route around and try to cut off the enemy with PB. Just spin in place at the exit that the enemy wants to use. This will give your allies a little bit of time to catch up. After keeping your prey pinned in place, finally smash him in the final seconds and let your team do the rest of the work.

2) Tremor Chase: Tremors works while rolling. Use this to your advantage. As the enemy is trying to flee catch up with Powerball and try to roll alongside them while dealing damage with Tremors. Ultimately hit them with your PB for the finishing blow.

3) Minion Hopping: When chasing down enemies, make sure to avoid any minions. They are the quickest end to your Powerball and chance of killing the enemy. Use Flash to hop over them and continue chasing. They will lose aggro on you so you will be able to catch your prey.


V. Saving Allies
Once again you are most powerful while your PB is still spinning, not after it has hit.

1) Be the Scatterer: You see your ally running with a bunch of enemies on his tail. Wait until there is a stretch of open land (don't do this in brush). Use PB and act as though you are about to charge into the group of them like a kamikaze maniac. As they prepare to switch targets, quickly change direction to the side. The fastest part of PB should be coming up. Though they are trying to change targets, there is no way they can catch you. Try to get them to attempt to follow you, while making sure you don't let them get close enough to get to you after PB wears off.

2) Use Your Knockback: If an enemy is closing distance on your ally, get between them with Powerball. This will knock your enemy back away from your ally and apply a slow, discouraging further pursuit.


VI. Invisibility Detector
This one is tough, and doesn't work all that well unless you have some knowledge that an invisible champion is trying to gank you.

1) Test the Waters with a Fake-out: If you think that an Eve or Twitch is likely trying to gank your lane, simply hit PB. If you don't immediately go and attack something, half of the time they will think that you know something about where they are... as though you had a ward near you. This will cause them to stupidly move into the brush... as though it made them more invisible than they already are. This tactic limits their possible location to a single line... perfect for a PB test. As you hit the insanely fast sweet spot of your PB, make a quick run through that brush. If you collide with something you have just successfully scared off their invisible champion. Congrats!

2) Protecting Versus Assassins: This bit is from iPlastic's Rammus guide:
Now, you have your precious cargo or one of them laning with you...your carries or allies...You have a shaco , an Eve and a Twitch against you (haha what a bad idea.) If they stealth, I will often use my power ball, and spin in a random pattern behind and on the sides of my team mates. There's a 65% chance, I hit pay dirt and pop one of the annoying *******, and then start barking like an alarm dog in vent or in game chat.

VII. Interrupting with Powerball
This is another piece from iPlastic's Rammus Guide:
Remember that Power Ball knocks your target(s) for a second interrupting their casting/movement. A good scenario would be Twitch and Shaco. Most of the time your team will stick you on detail to pull their massive damage off and onto you. When you guys focus Twitch and you guys can't kill him fairly quickly he will stealth. Once you taunt, count your seconds, then pop him with Power Ball. This gives you an added couple of seconds to guarantee his demise. Do not forget his Power Ball will work with channeled spells such as Nunu, Fiddle sticks, etc etc.


Note: This works by moving the enemy champion. Therefore it will only work to cancel channels that require the champion to stand still

Playing Rammus: Tips & Tricks

Using Puncturing Taunt if an enemy is near the tower, go to the opposite side of him, and use Powerball to push him further in can get an extra turret hit on your target.
Charging up Powerball in a bush before going in can be smart, especially for a gank.
Your passive, Spiked Shell gives you an adventage early on.
Rammus can harass. Taunting the enemy and activating Defensive Ball Curl will actually do a good amount of damage (against melee champions) due to both the fact that it increases your AD, and it hurts the enemy whenever he/she hits you. Only good early on.
You can surprise enemies by using Powerball and flash over obstacles or minions to hit them.
As I've said earlier, taunting an enemy (preferably a carry) AWAY from the enemy team, into your team for so your team to take the target down in 2 seconds can be very rewarding. 5v4 fight much?
Rammus is a really strong tower diver. With Guardian Angel and Defensive Ball Curl both activated, you can tank the turret pretty hard while both you and your mate dives.
GREAT ganking capabilities from the jungle.
You can travel over the map quite fast to help other lanes, or doing other things.
If you jungle, give the 2nd blue to your AP carry. This will give them a huge advantage in their lane.