Patch 1.0.0.112
Here's the condensed version of what changes affected Mundo.

• Spirit Visage healing is reduced from 20% to 15%

NUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU QQ it's a big NERF to MUNDO :( WHAT IS THIS. Sigh. We'll make do.
Here's the link to the patch http://www.leagueoflegends.com/news/release-notes-v100112

Patch 1.0.0.111
Here's the condensed version of what changes affected Mundo.

• Quicksilver Sash magic resistance increased to 56 from 48
• Randuin's Omen cooldown reduction decreased to 5% from 8%
• Spirit Visage cooldown reduction decreased to 10% from 12%

:( overall cooldown nerf. Quicksilver gives you better magic resist. That's awesome. Other than that it this nerf sucked.
Here's the link to the patch http://www.leagueoflegends.com/news/release-notes-v100111

Patch 1.0.0.110
Here's the condensed version of what changes affected Mundo.
NOTHING. The Quintessence of Fortitude nerf really hurt a lot of players but we use Focus Quints instead of Fortitude. Other than that nothing much notable about this patch.
Here's the link to the patch http://www.leagueoflegends.com/news/release-notes-v100110

Hey guys. Like the rest of the community here I've been reading a bunch a guides and I wanted to make my own to kinda just give back. To be honest, I've tried tank Mundo and I wanted to try passive Mundo which basically tank Mundo with more cd. The idea behind it is to proc your passive as much as possible while maxing out cd for your ult and such. Let's get on with it.

*UPDATE*
I've noticed a lot of views but not a lot of comments and I've even gotten 2 likes on Facebook. I didn't even know that there was a function for that LOL. Anyway playing more with Mundo, to be honest, I've not really noticed the passive too much. I focus more on the cooldown reduction my skill shots with Infected Bonesaw. I'll do some quick calculations for the passive. It procs for more regen scaling with max health. The modifier or the percent is 0.3% of your max health. Thus, 0.003 x an average of 3000 max hp. That gives you about 9 hp per second. 9 x 5 is 45 hp per 5 seconds. That's pretty good since I average around 150 hp per 5 seconds in a regular game. It's pretty significant in the long run. When you're running around. When I play with Mundo I usually only have to go back for my next item because between Maximum Dosage and my regen complemented by Goes Where He Pleases, I stay on the map for practically as long as I want to.

Okay so because there have been many views and no comments, or even votes, I feel inclined to show you my stats. I don't know how this will sway some people but I think that these are pretty good due to the fact that I never get a negative score with Mundo.

These are only my recent matches but I'll be updating as I go.

Regen=Regeneration
CD=Cooldown (maxes at 40% so whatever you buy after that for Cooldown is useless)
Champ=Champions or Enemies
Bluepill="b" or that blue circle thing when you press "b". It takes you back to your spawning pool and can be used anytime. One of the main highlights of LoL.
Cleaver="q" or Infected Bonesaw or that flying knife that you throw at people. It slows. It's awesome.
ad=attack dmg
ap=ability power
dmg=damage
Ult=Ultimate "r" or Maximum Dosage
boots=enhances mobility, a must for any champion
proc=fulfills requirement so that ability can be activated. Ex. Jax's counterattack. He has to dodge an attack in order to stun surrounding enemies.
CC=Crowd Control or disables. Ex. Amumu's AOE Ult.
AOE=Area of Effect.
Aura=an area around your character that is effected by an item. Ex. Will of the Ancients
dps=damage per second or characters that focus on their attacks rather than their abilities

Pros

• With good positioning you can last hit really well with Infected Bonesaw
• Infected Bonesaw does % dmg. Practically a built in Madred's Bloodrazor
• Great regen through his passive, Goes Where He Pleases
• Can stay in lane forever because of Maximum Dosage
• Farms well in later game with Heart Zapper
• Really short CD on Maximum Dosage which increases movement speed. Even shorter with 40% CD.
• Lots of hp = LOTS of regen. Goes Where He Pleases

Cons

• Squishy early on even with Runes and Masteries. Remember more health does not mean doing random gung ho **** and then trashing this build.
• Cannot run away well from 2 or more champs without Maximum Dosage. You can slow one all you want but the other is just going to catch up to you. Play it safe.
• Does not tower dive well.
• Can't soak up alot of dmg. You have to know when to run and when to stay and knife the enemy's face off.
• Unwise use of Heart Zapper during a team fight can leave you vulnerable with low hp.

Let's start with Masteries
I chose 9/0/21. The 4 I put in crit and ap is only to build up to Sorcery. This gives me 3% CD on my skills. That plus the 6% CD I get for building up to Intelligence gives me 9% without runes. This in conjunction with my Focus Runes lets me start the game off at a grand total of 19.77% CD. That's almost 20% CD at the beginning of the game!

I read from Restrictnine that building up to 21 on the Masteries is just a fad. The thing about this is after Intelligence you have 2 skill points left if you don't put your other one in Archaic Knowledge . That leaves you at 8/0/20. From here you have many choices as to where to spend your 2 points. You wouldn't want to put it into anymore Utility (except for Presence of the Master because those are either mana masteries or summoner skill masteries. In Defense you won't go very far. I wouldn't waste it on the extra Armor or Magic Resist. What's left is Offense. You can stack the 2 points into Sunder or one into the Burning Embers and the other into Archaic Knowledge or even Offensive Mastery . That's all up to you. Personally I like the 15% CD on Ghost and Ignite. The Magic Penetration from Archaic Knowledge will give you an edge in farming with Heart Zapper. Correct me if I'm wrong but I think this also helps with the % dmg dealt with Infected Bonesaw.

Let's try a little math. (I'll do it for you so you can just check the numbers or take my word for it.)
Let's say you start off with a Ruby Crystal and go to lane. You put your skill point on Infected Bonesaw. This gives you a CD of 3.2 seconds. It doesn't seem like much at first but you'll be throwing your Infected Bonesaw ALOT and the 0.8 seconds do make a difference. After you get enough for Kindlegem you bluepill back to get it. This brings your CD total up to 29.77%. Leaving your cleaver at 2.81 seconds to recharge everytime you toss it. This can be the difference between life and death when you're running away from someone. Every 2.8 seconds you turn around and hit him/her in the face with your cleaver. How great is that?

How about after you get your Spirit Visage?. It gives you an extra 2% CD. Let's focus on "e" or Blunt Force Trauma. This skill gives you a temporary dmg boost. The base CD is at 7 seconds. With the new CD at 31.77% with Spirit Visage your Blunt Force Trauma is brought down to 4.776 seconds. As long as you remember to press it, you can practically leave it forever.
CHANGED AFTER NERF IN PATCH 1.0.0.111 Spirit Visage no longer gives you that extra 2% :( It's a pretty significant nerf to Mundo. Anyway Blunt Force Trauma is now 4.916 seconds. That's the same thing as if you had that extra 2%. What it means is even after the nerf, you can still keep Blunt Force Trauma on. You still want to take Spirit Visage though because you need that 20% healing amplifier.
CHANGED AFTER NERF IN PATCH 1.0.0.112 Spirit Visage now has a 15% healing amplifier :(. NUUUU

With Randuin's Omen, your CD is NO LONGER capped at 39.77% which rounds to 40%.
This brings your Infected Bonesaw down to 2.4 seconds.
The CD after you toggle Heart Zapper is down to 2.4 seconds. You can practically toggle this on and off at will without worrying about the CD if you accidentally toggle it off
The CD for Blunt Force Trauma becomes 4.2 seconds. The further CD wasn't needed because the skill does not stack, the ability only refreshes. :(
FINALLY your ult, Maximum Dosage. It starts at 60 seconds. Even without Randuin's Omen, your ult will have a CD of 40.938 seconds. WITH Randuin's Omen your CD will be at 40% getting your Ult down to 36 seconds.
SINCE THE NERF PATCH 1.0.0.111 Randuin's Omen only gives 5%, reduced from 8%! This means that CD is capped at 34.77.

SOLUTION
Elixir of Brilliance!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Gives you 10% CD for 4 minutes! Buy this after your core every time you end up at your base. It pays off dearly as see above. Profit for only 250 gold :). You only need to kill one champ to get this money back. This is not recommended if you're losing. I would suggest that you finish your build rather than taking the 250 hit but if you're in a stalemate I suggest you drink this every 4 minutes.

But you want to max out CD right? Why don't you go for Randuin's Omen right after boots?
That's all up to preference.
If you find yourself in the middle of the fray with mostly dps characters, you might want to grab Randuin's Omen before Force of Nature.
If you find yourself in the middle of the fray and getting nuked by Annie, Ryze, etc. then get Force of Nature first.

PERCENT PERCENT PERCENT. This is item is all about percents. 10% CD and 20% healing boost. The latter is really important for your cleaver and your ult. The cleaver gives you half of the health used in the ability if it hits something. With this item, it gives you more health than half. More like 70% or so I assume. For Maximum Dosage it gives you more regen which is always good. This item is also good if you have healers on your team such as Taric, Soraka (it procs her ult and her q ability), and Janna (her ult). Other champions that you might benefit from are Zilean's Ult, Nidalee's heal, and Kayle's speed boost.

This along with Heart Zapper gives you 70% CC reduction. You won't be stunned for long!

*NOTE*
The next 3 items are interchangeable as in you pick! Get Force of Nature if there's a lot of AP champions on the other team. Get Randuin's Omen if there's a lot of DPS on the other team. Get Zeke's Harbinger if your team is inclined to group up and push. League of Legends is all about adaptability and I promise you that every game is different so get the items to counter your opponents.

AN AURAAAAAA of awesome. Regen. Helps dps, yours and others. Gives lifesteal. Lower's enemy armor. All around good item. Cheaper than most that give this many bonuses.

Gives you aoe slow centered around your champion, has 5% CD, health, regen, slows attack speed if you're getting bashed on by dps (20% chance). It compliments Mundo well.

MOAR MOVEMENT SPEED? YES PLEASE! PERCENT HEALTH REGEN?! OMGWTFBBQ. Most of all magic resist :) This item is the best on Mundo.

Want to be more annoying? Get this.

Alternative

Most useful item on Mundo after Spirit Visage. You need this item to take off ignites and wipes out all the other cc's you might be under. Especially in a ranked game. You pick Mundo and 4 out of 5 of the enemies have ignite. The other is probably Gangplank who crits you at the same time uses his healing reduction. Anyway best alternative. Take this instead of Zeke's Harbinger if your team doesn't demand the attack speed/lifesteal aura.

4% of health in Magic dmg on hit. Wow. With Arcaic Knowledge, it hits harder. You hit harder, they die faster, you win faster.

Nerfed recently and gives you 750 hp on revive but adds survivability. Gives armor and magic resistance and actually restores more because of Spirit Visage. The CD for this is pretty long but works well if you're fed.

2.28.11

• Updated Patch Notes
• Changed Description: Cooldown Reduction and MATH :) (Theorycrafting)

2.16.11

• Added Patch Notes
• Changed Description: Cooldown Reduction and MATH :) (Theorycrafting)
• Added Quicksilver Sash to Alternatives. Thank you Dotter :)

1.31.11

• Updated Introduction
• Added Credibility
• Updated Item Sequence
• Moved Changelog above Conclusion

1.25.11

• Added Changelog
• Took out Frozen Heart
• Took out Warmog's Armor
• Added Banshee's Veil
• Added Randuin's Omen
• Changed Runes: Glyphs from Resilience to Fortitude.
• Changed Runes: Quintessences from Fortitude to Focus.
• Changed Description: Cooldown Reduction and MATH :) (Theorycrafting)
• Changed Description: Masteries/Runes
• Changed Skill Build Order
• Changed Item Sequence
• Changed Pros/Cons

I'll be updating this as I go. An skill section will be included soon. Patch notes will be updated as well. Maybe more considerations to masteries and such.
Be warned I will report everyone who trolls by voting and giving some **** one word comment. Please leave constructive criticism. Thanks.
-Naijtyr