This is a direct copy of Bguggs's guide to Rammus on Leaguecraft - Master of Assists (by Bguggs [INN]). He has given me permission to transfer the guide over to MOBAFire in order to help more people with what I believe to be the best Rammus guide out there.
I did not write this guide. All credit goes to Bguggs, of the Leaguecraft community.

WARNING: MASSIVE GUIDE.
If you're a tl;dr type person, this is not for you. Fortunately, the table of contents does tend to help.

Please comment and try the build and strategies before voting. Thank you.
Last updated: 02/09/2011


Regarding Bguggs's thoughts on the above "item build" -
"I think it's a common misconception. There is no build. This isn't a build. There can be no build.
You can't "build" a tank with an item path. You can only list the options and scenarios in which they make the best choice. Every Rammus must learn to itemize each game as an independent unit. I think this is a limitation of MobaFire. It gets you thinking linearly, when LoL isn't a linear game.
HoG, Aegis, Randuin's... I mean, there are plenty of games when I get none of those items. The "build" listed on the cheat sheet is less than a suggestion, it's a snapshot of a possible game."

So TL;DR - Don't follow the above item build in every game. Use your own discretion. You're a tank, and you need to react accordingly to what the opposing team builds, not to some pre-determined item order.

Spiked Shell
Rammus gains additional damage as his shell becomes reinforced, converting 25% of his armor into damage.

This passive gives Rammus viable attack damage. He will never be Master Yi, but his physical attacks will be noticeable enough that the enemy can't simply ignore them. It does exactly what the tooltip says.

Conversions (+damage from armor only):
Doran's Shield = +2 damage
Cloth Armor = +4.5 damage
Ninja Tabi = +6.25 damage
Heart of Gold = +6.75 damage
Aegis of the Legion = +7.5 damage
Chain Vest and Sunfire Cape and Glacial Shroud and Atma's Impaler = +11.25 damage
Warden's Mail = +15 damage
Guardian Angel = +17 damage
Randuin's Omen = 20 damage
Frozen Heart = +24.75 damage
Thornmail = +25 damage
Easiest way to think of it: +20 armor = +5 damage


Powerball
Rammus accelerates in a ball towards his enemies, dealing 100/150/200/250/300 (+1) magic damage and causing 28/36/44/52/60% reduced movement speed to nearby enemies on impact.
Cooldown - 10/10/10/10/10 seconds
Cost - 80/90/100/110/120 Mana
Range - 300

** Hits all enemies in a 300 AoE.**
**Right clicking or A-clicking an enemy will not cause you to ram into them. Instead, you will stop just short of their model, in the position to autoattack. To ensure a hit, click a spot next to or behind them.**
** In order to hit Golem, Dragon, or a big Cho'Gath, you must collide with the back of their hitbox.**
** Powerball continues through Teleport: this can surprise enemies (props to Jemrin for the reminder).**
** Can be canceled at any time by pressing Q again.**


Defensive Ball Curl
Rammus goes into a defensive formation for 6 seconds, increasing his armor and magic resistance by 50/75/100/125/150 and returning 26/32/38/44/50[(+0.10)*(armor)] magic damage to attackers.
Cooldown - 14/14/14/14/14 seconds
Cost - 50/50/50/50/50 Mana
Range - 300

**You cannot use Powerball with Defensive Ball Curl(DBC) on.**
**Stops motion for .3(ish) seconds on cast.**
**Can be canceled at any time by tapping W again.**
**Damage return procs spell vamp (courtesy Civil Enemy).**
**Does not proc on Sivir's Ricochet or incoming Thornmail damage (courtesy Civil Enemy).**


Puncturing Taunt
Rammus taunts the unit into a reckless assault, reducing their armor by 10/15/20/25/30 and forcing them to attack Rammus for 1/1.5/2/2.5/3 second(s).
Cooldown - 12/12/12/12/12 seconds
Cost - 50/60/70/80/90 Mana
Range - 325

** Sometimes bugs a bit if at maximum 325 range. To ensure target switch get closer to enemy.**
** Remember enemy doesn't hit you immediately when you taunt. There is a brief time before they attack as they switch targets and move over to you or their attack cooldown timer ticks (this is important for tower aggro).**
** Cleansable.**
**Switches target of all minions except Baron.**


Tremors
Rammus creates tremors beneath him, dealing 65/130/195 (+0.3) magic damage per second to nearby units and structures.
Lasts 8 seconds.
Cooldown - 60 seconds
Cost - 120/120/120 Mana
Range - 300

**The damage is entirely independent of the animation, it will strike once per second (regardless of the apparent pulses) and a little bit beyond the range of the animation.**
**Continues during Powerball use.**
**The AoE hits through thin walls.**

Powerball Usage

Powerball

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 *****es, 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 (courtesy Frappe).