Could have gone in the other games section but meh:

http://www.l4d.com/

So basically this is a multiplayer game based on the CS:Source engine. It involves you and 3 friends escaping from zombie hordes in a teamplayer, zombie gun-fest. Not only is this clearly epic win made for TPF but the demo is out for those with pre-orders and the word is that it is good.

Its well worth checking out youtube for gameplay videos. As I understand it you have ~20 levels with each level divided into sections that end when you reach a safehouse. The gameplay is randomised in that you have what is called the Director (game AI) determining when, where and what zombies to spawn.

But I'm sure I had you at 'zombies' and 'teamplay'.