Please comment if you vote, every little bit of criticism, positive and negative helps.

THIS GUIDE IS A WIP. IT IS NOWHERE NEAR COMPLETE. IT IS CURRENTLY ON STANDBY. WILL UPDATE THIS GUIDE WHEN I HAVE SUFFICIENT TIME

I will add a dominion section as well as a tankier build.
Hey fellow LoL players! This is my first guide and covers a more obscure play-style of Nunu & Willump. Plenty of people play him as a jungler, AP, support and tank but few play him as an AP jungler. I found this very easy to pick up and my gut reaction made for a pretty powerful setup and tweaked it slightly for more success. It is a very gratifying role to play and can be done with relative ease, provided you have experience in the jungle. Please note that this is more of a build with a few tips, it is written assuming you have a general understanding of the jungler's role. If you do not, I recommend readingJungling 202: Beyond The Basics

-8/25/12: Preliminary guide published
-8/26/12:

Icons added for spells. Spell breakdown section added.
Explained thought process behind some of my choices.

-1/13/13: Got off my lazy *** and updated it for S3!

Though Nunu & Willump proclaims "Don't make the Yeti angry!" he is honestly more likely to cry, how would you feel if your diet consisted of golems and wraiths? When you gank a lane, Nunu is one of the few champions that you want enemy minions around. why? Nom to heal. Willump will love you for it.

Pros
-Got better with S3!
-Is always viable, no matter the stage of the game.
-Minion true nuke Consume with a base + 1:1 heal off of AP!
-Insane ultimate ability Absolute Zero , max of 1125 base damage plus 2.5 magic damage per AP!
-You can take a step while channeling Ult to deal a lesser degree of damage sooner.
- Ice Blast with Blood Boil is a fantastic chasing tool set
-Great early game passive Visionary , allows you to give every blue buff after the 1st one away.

Cons
-An interrupted ult does nothing whereas a canceled one does damage.
-Relies on your team for ganks early since he only has 1 reliable nuke.
-No natural escape mechanism besides a movement speed steroid.

Great spells
- Take this, free 10g on cast, down 1st blue in 4 seconds as well as the ability to steal/secure enemy buffs, dragon and baron.
- Pretty much a must-have. Great escape and causes mass confusion if you use flash and your ult in conjunction.

Decent spells
- A great spell, but the CD is long and the animation is visible to enemies, not really worth having unless you like to be everywhere at once.
- As a jungle without hard CC this is a great spell to have, but I normally skip it because Nunu slows stack with Rylai's.
- Decent if you slow them, overtake them then make them stand in your ult, also decent for escape, but your Blood Boil acts as a soft ghost on a short CD
Bad spells
- You are the jungle, then again, Nunu's Q is a smite.
- You can already heal yourself.
- You are not the support, a ward will suffice.
- Your passive should save you mana in the jungle.
- Not really needed, only fools focus you anyway.
- Nunu doesn't die. Don't be bad.

Due to time restraints I am using old rune names, I will update them in the future, you all should still know what they are.
Quints
I take Greater Quintessence of Ability Power or Greater Quintessence of Gold, using Greater Quintessence of Magic Penetration or Greater Quintessence of Health are also solid picks.

Marks
These are pretty much personal preference. I like Greater Mark of Attack Speed in the jungle, it synergizes well with your passive. If you want to eek out a little more damage, take Greater Mark of Magic Penetration.

Seals
Greater Seal of Armor
Take flat armor armor, you need it.

Glyphs
I take Greater Glyph of Magic Resist or using the scaling Greater Glyph of Scaling Magic Resist, personal preference here.

This comes down to play-style. I personally like doing a little more damage but doing 9/21/0 or even 0/21/9 is perfectly acceptable. If you have a mid that conserves mana or gets a chalice as part of their core (like Galio) you can go for the buff-jewing utility mastery.

This guide is focused on being a s hearty AP jungle. It is not meant to be a substitute for a true tank because I like to do damage, his ratios make him do LOTS or damage, with blue buff, you are spamming Ice Blast. Your job is to be annoying, chase the fleeing and feed your carries while not dying. My build focuses on that.
Item Rundown
Mercury Treads- Because CC makes the game.
Rod of Ages- A spectacular item, provides great sustain while leveling and caps out with respectable stats, most importantly health. 1st core item.
Rylai's Crystal Scepter- Though people bicker about slows stacking, the simple fact is, they do. It is not additive, there is an equation and a rule set to it, but item slows and spell slows stack. It also gives more HP. 2nd core item.
Abyssal Mask- I love this item, it lowers their MR while increasing yours. Great item, makes you a little more tanky.
Zhonya's Hourglass- Also another one of my favorites, again, makes you tanky with a great amount of AP and a fantastic on-use that puts you into stasis for 2 seconds.
Rabadon's Deathcap- Take this is your team has a support that is already dropping wards.
Void Staff- I don't ever take it, but if the enemy is building tons of MR and you aren't dying, you should consider it.
Guardian Angel- If you are dying, please use consume on CD and consider this.
Liandry's Torment- This item nearly made me wet myself on the spot. The DoT does double damage because Nunu's snow balls slow!
Twin Shadows This item has some innate tankiness, is build from a Kage's so it can pay for part of itself and has slow 2 people from super long ranges. I love this item.
Banner of Command Enemy at baron? You just derping around or your team is holding their own in a fight? Plop this bad boy down and Blood Boil the promoted minion and you have a BAMF pushing machine! (Note that you do not actually plop it into the ground, you are not Jarvan.)

Skill sequence is pretty much common sense, you take your Consume at level 1, then max out Ice Blast by level 9. Take Absolute Zero at 6, 11 and 16. Then prioritize Consume over Blood Boil unless your sustain is fine.

This section will have basic info and some of the nitty-grittys about his kit.
Passive After 5 autoattacks, Nunu's next spell will have no mana cost.
This ability allows you to spend almost no mana in the jungle as long as you have some patience, if you have blue, this passive sucks. But it also allows you to give the blue buff to others.

Q (Active): Nunu's Yeti takes a bite out of target enemy minion or neutral monster, dealing true damage to it and restoring Nunu's health.
Cost: 60 / 60 / 60 / 60/ 60 Cooldown: 16 / 14 / 12 / 10 / 8 seconds Health Restored: 125 / 180 / 235 / 290 / 345 (+1.0 per ability power)
I love this ability, it is a great sustain tool that serves as a heal and true nuke. This is what lets you keep your health topped off for those level 2 ganks as well as healing you for nearly 900 late game.

W (Active): Invigorates an allied unit by heating their blood, increasing their attack speed and movement speed for 15 seconds. If cast on an ally, Nunu also gains the effect of Blood Boil. Cost: 50 mana
Attack Speed: 25 / 35 / 45 / 55 / 65 % Movement Speed: 11 / 12 / 13 / 14 / 15 %
Another great move in his kit, this makes him decent at punching down towers, give it to your carry and they will benefit. The sweet thing is, no matter who you use it on, it gives you the bonus too. Its the gift that keeps on giving!

E (Active): Nunu throws a ball of ice at an enemy unit, dealing magic damage and slowing their movement speed by a percentage and attack speed by 25% for 4 seconds.
Cooldown: 6 seconds Cost: 75 / 85 / 95 / 105 / 115 mana
Magic Damage: 85 / 130 / 175 / 225 / 275 (+1.0 per ability power)
Slow: 20 / 30 / 40 / 50 / 60 %
This is your bread and butter. Fling it at their squishy every time you get the chance, it hurts and slows. Guess what? Since it is a spell, it stacks with the slow from Rylai's Crystal Scepter Sadly the combo doesn't slow as much as you think it would, but it is still substantial. With the CDR from blue buff, you can potentially keep the enemy perma-slowed. Take that Ashe!

R (Active): Nunu starts channeling and begins to sap a large area around him of heat, slowing all nearby enemies movement speed by 50% and attack speed by 25%. After channeling for 3 seconds, he deals massive damage to all enemies caught in the area. It can be canceled early for less damage. Cost: 150 mana Cooldown: 150 / 120 / 90 seconds
Magic Damage: 625 / 875 / 1125 (+2.5 per ability power)
Ahh, the pièce de résistance. Possibly the most devastating spell in the game, capable of getting penta-kills on its own. Though this spell takes gut reaction to use right. It is better to cancel the spell mid-cast for some effect that you being CCed and it fizzle out, doing absolutely nothing. It has a built in slow of 50% as well as an attack speed reduction of 25%. So you get a mass exhaust followed up by a 2000~ damage AoE nuke.

This is my jungle route. The basic order is Blue, wolves, chuck and Ice Ball at mid, wraiths, red, the block heads then chuck an ice ball bottom. I would not gank 2c early unless you plan to invade the enemy jungle or else you could end up under-leveled. I normally go back to base when I have enough money to build my Mercury Treads and Catalyst of the Protector. I try to do dragon at least once by the 20 minute mark, it is easiest after a successful gank bot so the fellows down there can help you out.

Play Video
This is clearly an extreme example, but that is one good way to use his ultimate, it also shows the sheer power of it is used correctly. The enemy Nunu also wastes their ult.

Nunu is great at chasing people down and chucking snow balls at them, you will probably need your team to help you out with the kill though.

Overall I think I do pretty well with this, I'll continue to post more games here if I find them exceptional. Feel free to comment your games with the guide and I'll post them as well :3

Nunu should NOT be used against a CC heavy team
In general, I avoid playing Nunu & Willump if the enemy has:
Cho'Gath-He has 2 disrupts that will screw you over if you try to ult.
Kassadin- Not only can he warp out of your ult, he can silence it too.
Janna- Her ult is obnoxious.
Alistar- I won't even bother explaining.
Vayne - Not only can she tumble out of your ult, she can pin you too a wall and unleash a barrage of shots with bonus true damage. She sucks.
Anyone else with a short CD reliable disable.

Nunu is good against
Tryndamere- As long as you have the AP you can out-sustain him and zone him hard, you kind of need too, if he gets farm, you lose your edge.
Kog'Maw- He just DIES, make sure you can survive his passive though.
People that have trouble running away and people with weak early games.