Hello everyone, and welcome to my first build! (hope not the last one) I'll do my best to explain how to play with MUNDO. The reason I write this build is because I've seen a lot of other MUNDO builds, but I think ,they are not so effective. So please, if you like the purple guy smashing everyones face and heals like a mad try this out :-p

Pros:

Good harasser
After mid game good pusher
With ulti he is one of the thougher tanks
Late game almost invulnerable but still extremely good dmg

Cons:

Squishy early game (can be avoided if playing defensively and/or laning up with a support champ with heal)
Requies skills (so if you can play only with Vladimir Akali Anivia Mordekaiser Garen and so on you shouldn't try him)
Requies practice (if Mundo is your first champ with a skill that requires aiming you'll hate me and downrate my poor guide )= .)

Infected Cleaver: This skill is your main damage, harassment tool, and it helps in last hitting too. You have to max it first, and use always ,if you have chance to hit an enemy champion or last hit a minion.

Burning Agony: This skill isn't important for you early game. You don't need any points on it until lvl 4 ,because it costs a lot of health. Don't use it early game, just if you can kill someone ,because it deals on lvl 1 a sunfire capes dmg. The reason why is this skill so good ,is the CC reduction. With the 35% CC red from mercury treads it's 70% ,so you're kinda unstoppable. In early game if you see something follows you (taric stun ,sion stun ,nunu slow) you can activate it for a second ,and then deactivate ,so it don't costs so much health but lets you run away faster.

Masochism: This skill helps you a lot early game ,because if combined with cleaver, you just can't miss a last hit ,but if you use cleaver for ar***ing enemy all the time it's still a good last hit skill. The reason I max burning agony first instead of masichism is that they deal the same dmg on the same lvls ,but the CC red on burning agony is much more important.

Sadism: The ultimate, that makes you a true tank, you can use it to stay longer in a lane ,or to being almost invulnerable in a fight. Just care about thoose ****py ignites. ( they hurt! =( )

So let's talk a lil bit about runes!
I get armor penetration marks ,magic resist glyps, armor seals and health quints. I couldn't test enough this part of my build ,but I think ,if you search for some informations about runes you'll know what else can you pick.

WTF armor penetration runes but magic penetration masteries are you mad?

Well, no, I'm not. You must get armor pen marks ,because basically MUNDO deals physical dmg. (expect burning agony and cleaver) With arm pen you'll just deal a lot of dmg ,and still can't die ,after you buy your atmas impaler. And I found magic pen in masteries better as some extra xp with health and mana( for MUNDO?!?) regen and 1 gold per 10 sec.

Usually I get Clairvoyance and Flash.
This is another thing where you can do what you want. I get clayvorance for more map control ,and checking baron/dragon/buffs. It's also good to check the bushes near your or your other teammates lane if an op midlaner or jungler is on SS.
Flash is because I love it and I pick it for all my champs all the time :)
If one of my teammates already has clayvorance ,I usually pick teleport.

Don't get theese spells:

(with 70% CC red? what do you want?) (Note: I tested it ,and it doesn't clean ignite so it's useless for Mundo)

Spells some ppl prefer but I don't:

(You have your ulti to run faster. And flash is much more better than ghost in my opinion. Cause almost all the champs has some slows/stuns/exhaust or another ghost or haste for themselves to chase you or run away. If someone want to run faster than you ,then he'll)
Exhaustsize=32 (I just don't like it. Mundo isn't a nuker who deals a lot of dmg in 0.5 sec and must have an exhaust until his spells recharge or something.)
(It don't help too much, if a group of enemy champs trying to push a turret this spell won't stop them, especially late game. + it has a long cd. If you rly wanna pick a spell wich can save your team a turret ,then pick teleport.)
(Early game it don't deal good dmg, late game not so good too. It's nice to ignite someone with huge heal (warwick ,master yi while casting heal ,or a mundo while ulti) but I don't think it's Mundos job to do this)

This is the most important part of every guides, and the reason I write this guide. I've seen Mundos hanging around with, starks fervors, boots of swiftnesses, ap items etc...

The beginning

Doran's Shield or Regrowth Pendant+ Health Potion? o.O

Doran's Shield gives 120 health, 10 armor and 8 health per 5 sec.
Regrowth pendant gives 15 health per 5 sec and you have enough money to buy a health potion too. + you can get your Warmog's Armor faster.
This is really situational. I usually pick Doran's shield, because Mundo is extremely squishy early game, and most of the time when you have a jungler you must solo. I just died under turret sometimes ,because the enemy killed me on solo lane 2v1 on lvl 2-3.
You must choose your first item smart. If you go solo lane, then just pick the Doran's shield. If you lane up with someone vs 2 enemy pick Doran's too. If the enemy has a solo lane, and you're on it with a teammate ,you can get Regrowth pendant, because the more survivability isn't so necessary ,and you can rush Warmog's faster. And if you're on lane with someone who has heal (Nidalee, Taric, Soraka, Kayle...), then you can get regrowth too. However it's not so good idea, because most of the time your team'll have squishy mages, ad carries and so on, and they need that heal to stay on lane early game much more than you.

I usually rush Warmog's first. (so I can start getting thoose stacks on it and makes my ultimate ,wich isn't so effective early game a little bit better) The best is if you can get a Giant's Belt first time you go home and the rest second time, or rush the whole Warmog's first time :)
After this ,you want to buy normal boots. Tier 1 shoes combined with your ultimate must be enough until you get Spirit Visage.
Most of the time I buy Mercury Treads ,because the mr and the CC red. If you facing with a hard AD team with not so many CC you can buy Ninja Tabi.
Theese items are the core of the build ,so you must have them. After this time there's a lot of items you can buy. The items in purchase order are vs a balanced team.

Vs AD team:

You can get a Thornmail instead of Force of Nature.

Vs AP team:

You can get a Banshee's Veil instead of Frozen Mallet.

Other items you can get:

Sunfire Aegis: If you don't like Frozen Mallet ,you can get this instead of it. Gives you a little bit lesser hp ,but if you need more armor vs a strong ad team, this is a good item. It also improves your DOT around you, so with burning agony you'll deal 140 magic dmg per sec.

Thought he is a tank ,he can die, if the enemy has the right counters. If you play Mundo good ,then most of your deaths is in early/mid game and in lategame you have 0 or 1-2 deaths. But let's talk about what can kill a Mundo:

Ignite

This is the best spell to counter Mundo, because it reduces all healing effects on you by 50%. There are also other spells reducing healing effects on you by 50% (like Katarinas W+Q combo). If you see you have any kinda healing reduce effects on you, you must know, how long can you survive without your ultimate in the middle of the fight. This time you must run away a little bit to don't get focused and throw some cleavers from behind. It's good to have an other tank or offtank in team, because mid-game teamfights can be lost because an ignited Mundo.
(NOTE: I checked ,katarinas upgraded Q reduces your healing effects by 50% for 10 seconds! Your ultimate lasts for 12 seconds. This means you can heal about 30% of your max hp instead of 60%. I think there's no other spell wich has a that long healing reduction effect ,so beware with Katarinas! [nerf her Riot!])

Madred's Bloodrazor

Well, some guys may think I'm a hp stacker and if the ad carry/carries buy Madred's it's over. But I don't think so, and let me explain why:
It deals 4% of your max health as magic dmg with each hit, but you'll have even without Force of Nature at least 100 magic resist mid game (mercury treads, spirit visage, runes). 100 mr means 50% less dmg so it's only 2%. So it means 20 extra dmg/1000 hp.
+ they need to hit you as much times as they can. This means they must buy more attack speed items, so they can't buy any kinda defenses, that means they'll be squishy and just can't survive long enough to deal the dmg to you.
If on some heroic ways they just can't die and do everything to kill you, you can just start running and throwing cleavers to slow them, and your teammates can kill the others, and even if you die, they can kill them too.

So if you don't have a jungler, or someone else want to go to solo lane it's a good idea to pick an optimal laning partner. You can choose
-a support laning partner who heals you ,so you can ignore the enemy harassment
-a ganker hero, with good CCs and a lot of dmg, so you can harass the enemy, deny xp by sending them home often, or even get a few kills.
-or an OP hero with both :)

Ofc I can't try all the champs, because every game is other and can't plan it, but here are some excellent laning partner:

Nidalee (Her spear + your cleaver working on the same way ,so if you can hit someone it's like a double strike. + she can heal you.)
Sion (He is a really op champ, has a long stun and an extreme dmg. If he makes a stun+shield combo ,and you go in too, slowing the enemy with cleaver, and both of you use his improved auto attacks you can easily kill even the hardest guys early game.)
Taric (Heal+long stun. He is good too.)
Akali (She can harass good and keep the enemy far away from his minions so they won't get xp. And you both are deadly after lvl 6 so when mid-game starts you can get even a double kill.)
Maokai (With his passive he can regen hp with melee attacks after 4 spells casts near him. Because you can spam your cleaver and masochism he can heal even from almost no hp to max with your help. And with his 2 CC you can get a kill or two.)

Farming with Mundo is no problem. Even if you spam your cleavers always to harass the enemy champs you have Masochism to improve your auto attacks. And if you really missed a minion you can activate burning agony for a second to deal the finishing blow, and then deactivate it.
When I go to solo lane and do my job, usually after I have my ultimate we can get a kill with the jungler. If you stay on top lane and leave it only to buy Warmog's and for ganks/teamfights until the first turret is down, you can be the first one reaching lvl 11. And if you doing great and die not so much times you can be the first reaching lvl18. This is a huge bonus for you and your team, and because you have your ultimate to heal, after teamfights you can go to push lanes/farm minions/jungle so money wouldn't be a problem too.

So, Mundo is squishy early game, but invulnerable late-game. You won't have any problems if playing passive early game and just harassing the enemy team with cleavers. You must know, when to use ultimate, because it's not just a good healing source, but also gives you movement speed, so if there are any ganking opportunities you shouldn't waste it. And sometimes the enemy team just waiting for you in fights to use your ulti, so they can pop an ignite or any healing reduce spells on you.

Thank you for read my first guide, hope you enjoyed it, please vote if you can, and let me know in comments if you have any problems =)

Here you can find all the updates I made after published this guide:

2011 apr. 2: Added pictures to items spells etc. so you can now enjoy the guide much more!