Hello and welcome to the guide on how to build Alistar as an amazing tanking beast.
If you try to find a decent tank build for Alistar it is not going to be easy seeing as how most builds for him are AP based and a lot of people just aren't good enough of a team player to build a character that exists solely for getting assists and absorbing damage for your team

Mana , magic resist , health and a bit of magic penetration makes you a gooooood tank.

Again I like building Alistar as a Pure Tank so the decision to put 21 points in defencewas easily made. If you want to make it a bit easier on yourself to get kills then put the other 9 in offence, I choose to put them in utilitywith a focus on mana so i can spam my skills more and get more assists.

Flash, flash, flash, FLASH ! Flash is the most amazing spell for Alistar, you can flash behind someone close to your turret, throw them in the air(pulverize ) and then push them right towards your turret which stuns them again for a little while. Also when chasing enemies you can flash close enough to the enemy to throw them in the air and give you and your team mates an easy kill. For the second spell you can choose either ghost or ignite, I choose ignite to easily get the first blood and to get that pesky little last of health away from your enemy. Both work fine though.

To start off we take the ruby crystal . Seeing as it costs 475 gold which is exactly the amount you get to start with it seems like a good choice to survive early level 1 fights by giving you that little extra bit of help that you can surely use. After that you can get the heart of gold which gives you a decent amount of gold seeing as how this tank builded Alistar isn't exactly great at farming, this way you can still get a bit of gold even without getting to many minion kills. Alistar needs a lot of speed to get around and behind his opponents so i definitely recommend you get the fastest boots in the game: boot of mobility . One could argue that when you get attacked with boots of mobility that your movement speed drops back down and Boots of Swiftness would be a better choice seeing as how it gives you a slower but a constant movement speed bonus. I find from personal experience that Boots of Mobility are simply the better choice and often when you really need your speed it is when you are trying to gank someone or rush somewhere, and at that moment you aren't being attacked so you are a lot faster! After that it is vital for you to get Randuin's Omen which has a passive that can hurt a lot of enemy players with a great crowd control affect once again. After that I try and decide whether I am receiving a lot of magic damage or a lot of physical damage. Force of nature is a good choice to counter the magic damage but make sure you get the negatron cloak first seeing as that is the thing that is doing all of the magic resist. If you are experiencing a lot of physical damage from by example Ashe or Yi you can get the Warmogs Armor . Please note that this is a stackable item that gives you more and more health when you stay alive longer and get more kills. If you are constantly dying it might be a better choice to go straight to Banshee's Veil . You will probably never come to this but if you are in a long game a Frozen Heart is always useful to get in late game seeing as how it gives you a lot of armor and a very big amount of mana which you can then use to spam your skills the entire time. If you are in an extremely long game you can also decide to get the Guardian Angel item to revive you even in the off chance that you should die. That is about all you need to know to get good items for your Alistar.

Pulverize is the first choice you should always do since it is just devastating at level 1. It stuns an enemy for so long that it is almost impossible for him to survive when you are paired with a lanemate that also has an ignite summoner spell. For your second skill, if you are still full health and the enemy is barely touching you, you should go for headbutt and try to flash behind an enemy and throw him back to your team mate or preferably your own tower. If you are harassed constantly by those annoying ranged champions (which will happen quite often) then you should get Triumphant Roar
first and heal yourself, your lane mate and your minions as much as possible but always try to have enough many to execute your pulverize headbutt combination. After you have all three skill unlocked it is a matter of making pulverize and triumphant roar
as strong as possible depending on how much you need the one or the other. Headbutt doesn't level up as well as the other 2 do so you should upgrade headbutt as late as possible in the game. Of course as is with all the ultimate skills you should upgrade Unbreakable Will every chance you get (Level 6,11 and 16).

PROS:
Alistar is fantastic at team play and you can pulverize and headbutt any enemy back to your tower to get an easy kill or assist.
He is great at absorbing damage with his ultimate that reduces 75% of the damage done to him and which removes all of the possible stuns or debuffs that are put on him (such as Ashe's pesky big arrow).
Alistar is also great at tanking a tower in late level. So starting from level 14-15 you don't even have to wait for minions, you can just get one or 2 team mates to help you and take on a tower on your own.

CONS:
Alistar is no good at farming. Last hitting minions helps but most of the time you just need to get some XP with Alistar and depend on your heart of gold to do the rest for you.
Alistar built this way does practically no damage. Should you come across any tanks or strong champions you can do practically nothing to them. That's why ignite can be such a great spell for him but you should just use Alistar to keep enemy's in their place until your team mates arrive. Do not try to take on a champion on full HP by yourself. You will lose unless you are extremely fed.
Alistar hates 'Silenced'. Against enemy's like Fiddlesticks it is hard to play seeing as how he relies on his spells to do crowd control and survive difficult situations. Fiddlesticks makes it impossible for Alistar to do anything and you will die quite soon. Make sure you have a few team mates around you when you encounter a Fiddlesticks seeing as how your baisc attack doesn't do a lot of damage.
It sometimes sucks that you can headbutt people further away from you. Especially in chasing it can be a pane in the *** but if you have enough experience and try a few things out it shouldn't happen that often.

Alistar built this way is an assisting machine. This build is made to feed your lane mates and make sure that they can win the game and kill enemy's while you stand there and soak up the damage. Note that it is not bad should you get a negative KD ratio but a very high assists count. Better you die than a team mate that had a 5-0 before their death. Team play is the way to go with a tanky Alistar.

In a lot of youtube videos and other examples for playing Alistar you can see a lot of people failing (using spells with no flash, headbutting away from you, missing pulverizes , ...).
Alistar is a combo machine:

Combo 1: Flash-Pulverize-Headbutt - -
Flash close enough to an enemy to throw him in the air with pulverize , instead of attacking him straight on try to run past the enemy and wait for the stun to wear off, while your team mate is attacking the enemy you can headbutt him back when the pulverize stun has finally worn off. This way you can do the maximum of damage to an opponent without eventually cancelling any special skills or ultimates that your team mate was performing at that time.

Combo 2: Headbutt-Pulverize -
If you are chasing an enemy or you chose want to harass him a little or get the kill when he is low HP, you should try to use headbutt and then very quickly do a pulverize , instead of headbutting him away you will throw him up in the air at the end of your headbutt . Notice that this takes a bit of practice and when you are not really good at it, it might cause you to throw people further away from you instead of getting the kill.

Combo 3: Unbreakable Will-Pulverize-Headbutt - -
When you are near a tower and an enemy is taunting you and your team mates. Pop your ultimate and tank the tower attack while throwing the enemy in the air and pushing him back towards the rest of your team mates and making sure that they get they kill they have all been waiting for on that pesky little tower hugger.

A last handy trick when chasing an enemy or try to run away from one is using headbutt on a minion, it is very useful to get you closer or further away from an enemy should you chose to do so. Flash can do this quite good as well but it has a pretty damn long timer on it so that can be a pain in the (head)butt sometimes :).

This concludes my build of Alistar the mighty Pure Tank. I hope you enjoy it, get good results with it but most importantly soak up a lot of damage for your team and get a lot of assists.

A few examples of good games with an Alistar.

11/03/2012: With the current patches Alistar gets buffed again but he may become a bit more mana hungry. Rushing the sheen only can help you with that. An other viable alternative is going philosopher stone but make sure you don't become too 'supporty' this way.