Warwick was my first champion in LOL and I've grown to love him. I've fooled around with a lot of builds, but I've finally settled on this one. After alot of experimentation, I decided that Warwick is almost unstoppable when tanky. He loses alto of utility late game when built as pure DPS, but this build will allow him to be a solid contributor from start to finish. This is my first build guide so be gentle :) Enjoy! Please vote and comment!

PROS

One of the best junglers in the game.
Amazing health regeneration.
Stress free early leveling.
Good at buff/dragon/baron control.
Great at ganks.
Low CD ult with a stun.
Can be tanky as well as physical dps.

CONS

Prone to ganks.
Not good laning early game.
Not so great in team fights.
Squishy early game.
Cannot die while jungling or he won't be very effective.
Less utility late game unless tanky.

I think 9-0-21 is the best way to go. It gives you a nice mana boost which is necessary for spamming Hungering Strike. The experience boost makes you hit six faster, and the movement speed bonus helps to initiate ganks. Extended buff duration might be the most useful skill in this mastery tree. It makes jungling a breeze and the last thing an enemy wants to see is a Warwick with Golem AND Lizard buffs coming at him from the jungle. The rest are for ability cooldown (Again, helping to spam your Q) and summoner spell cooldown.
I have also seen 1-19-10, and this is very effective if you want a little more durability early game.

Greater Mark of Desolation Provides armor penetration, which allows you to shred through the jungle (and enemy champions).
Greater Seal of Evasion Dodge chance is such an underrated stat. Gives you more survivability in the jungle and when being attacked.
Greater Glyph of Cooldown Reduction Cooldown reduction is great on Warwick. It allows you to spam Hungering Strike and makes your ultimate come off of cooldown much faster, which creates ample opportunity for ganking.
Greater Quintessence of desolation More armor penetration.

This passive restores health when attacking. Gives Warwick great sustainability and allows him to be an efficient jungler.

Love this ability. Low CD, decent damage output, and it heals you for ALL of the damage dealt. Good range too for getting that last hit on a fleeing champion. Once you have blue buff or a healthy mana pool you can spam this puppy, which in addition to being a nice technique for dealing damage to champions, is a great way to last hit minions.

Makes Warwick one of the fastest junglers in the game. Also good for quickly clearing a wave of minions. Nice tower pushing skill because it gives your fellow champions a small attack speed boost as well.

I only recently found out how much i love this skill. Great for chasing and ganking. The only caveat is that if you are sneaking up on a low health champion with this toggled, your element of surprise is ruined.
KEEP THIS OFF BEFORE INITIATING A GANK.
It can make all the difference between an easy kill and the target scurrying back to their tower.

Great ult. Its low cooldown allows for a nearly endless stream of ganks. Suppressing a target while you take out half of their health is a beautiful feeling. Great range to catch fleeing champions. A nice plus about this ability is that it stacks on-hit effects like Madred's Bloodrazor and Black Cleaver, which can make it even more powerful. A word of advice on this one; Do NOT initiate with Infinite Duress. It is much more useful to catch a champion after already having dealt damage to it. Especially if you are not at full health, it is better to use this later in a skirmish to regain your health. If you pop out of the bushes leading with this bad boy, you're losing a lot of its utility. Also, coordination with your team mates makes this even more effective. Trapping an enemy in Absolute Zero to ensure full damage is a great way to ensure a kill in a team fight. This works for many AoE abilities, and can give your team an advantage in team fights.

Your Q deals a lot of damage as well as providing a ton of healing. The W ability is definitely useful, but your E ability is a great way to chase down enemies to get that last hit.
The skill order I suggest is maxing R -> Q -> E -> W.

Essential

This is a jungler's bread and butter. Makes jungling fast and effective. Get it and forget it.

Rcommended

Great for dealing that last bit of damage to kill a champion. Can also be used on jungle monsters if you are struggling.

Love exhaust, and sometimes trade a mastery point of Deadliness to put another in Cripple to use this ability. Good way to initiate while you hack away. Good as a defensive escape skill too.

A lot of people like this skill on Warwick. I don't particularly like it, but if you find it useful, it is a great skill to have. Flashing through a wall to escape or flashing to get your [infinite duress] in range is very effective.

Other Useful Spells

Great chase and escape skill, but [blood scent] renders it unnecessary.

At some point, you might get focused. Escaping crowd control in a team fight can keep you alive.

Cloth armor and health pots are needed here. Sometimes i like to buy a ward, which I will explain in the next chapter. Madred's Razor and Boots are standard for a jungler's first purchase. I then buy Mercury's Treads and Aegis of the Legion. These provide magic resistance, armor, and crowd control reduction. The aura on Aegis of the Legion also helps in team fights. Next I buy Sunfire Aegis. At this point, you will be pretty beefy. Spirit Visage is a great item here because it decreases your cooldowns AND increases self-healing. This gives Warwick great sustainability. Next is Banshee's Veil, which will give you even more health and protect you against those pesky Karthus and Ashe ultimates. Finally, completing Madred's Bloodrazor will give you an extra boost of power in the late game. DON'T FORGET TO BUY ELIXERS LATE GAME. You can buy these even if your build is complete. If you have extra gold they can give you an early (in only temporary boost). As the jungler, you'll be expected to buy and use wards. When you have leftover gold, ward strategic points such as dragon, baron, and chokepoints at lane entries.

As I mentioned earlier, you will be held responsible for the majority of the warding. I was going to summarize warding strategy, but then i came across OddSquad's guide to map control and awareness. This is the most comprehensive guide i have seen and i can't explain it any better. Credit where credit is due, great guide OddSquad.
http://www.leagueoflegends.com/board/showthread.php?t=527054
Map awareness and control is vital when jungling. It makes your team much safer and gives you a huge advantage.

At lower (Not ranked) levels, I like to ward their blue if they have a jungler. DO NOT GO ALONE. You will get ganked. You will die. You will be told to uninstall. Get your whole team to escort you. You may even get a kill or two out of this. If not, you or your team may be able to gank him later (especially if a teammate teleports to the ward).

I like to start jungling at my Blue Golem. This buff makes jungling pretty easy. Put your first skill point in your Q ability, and ask a teammate or two to accompany you, and have one leash (attack it so it focuses them, while they run away you attack it without receiving damage) the Golem. If another teammate wants to deal some damage to the Golem, it goes much faster, with less loss of health to you. Smite the Golem when you are sure you will not get ganked. Use Hungering Strike to stay alive. Use a health pot if necessary, and finish off the little guys with Hungering Strike to regain health. You'll hit level two, so put the next point in hunter's call.

Move on to the Wolf Camp. Start on the largest wolf with Hunter's Call, and use Hungering Strike as necessary to stay at high health.

Finish them off and move on to the Wraith Camp. Focus on the blue one and use W and Q to finish them all off. Put your next point in your Q.

At this point you might be at low health if you only had two health pots. If you have enough, start on the Lizard Camp. Hungering Strike and Hunter's Call will keep you alive. Smite the biggest lizard and kill the camp.

Now kill the Small Golem Camp to regain health.

At this point you should be level 4, have both buffs, two points in Q, and one in W and E. Check the middle and bottom lanes to see if a gank is available. If it is, announce who you will gank and wait in the bushes to strike.

If no gank is available, either go into the opposing jungle (if the other team does not have a jungler) and clear the other Blue Golem and Wolf Camps. Many junglers leave one lizard alive at the Blue Golem and bLizard Camp to prevent a new buff from spawning. But if you'll be back to steal it when it respawns, go ahead and clear the camp.

If they have a jungler, clear your own Wraith Camp and Wolf Camp and head back to base.
Here you should buy madred's razor and Boots.

Here is where the fun starts. Check all lanes for ganks and attempt a gank at one of them. This is Warwick's role until late game team fights. Cover a lane if a teammate needs to blue pill it out of lane. When you are level 7 or 8, attempt to dragon with teammates. Warwick used to be able to solo dragon at level 5, but that stupid patch ruined all that fun. Continue to monitor dragon all game, as it should be warded. Continue to gank and push lanes until you and your teammates are arounf level 15. This is the time that you and your team should consider killing baron. Warwick can solo Baron with a full build at 18, but the team approach is the way to handle this.

As the tank, you will be involved in most team fights. Don't get isolated from your team. Get baron, win a team fight, push a lane, repeat, WIN!