I'm at a LAN right now and we discovered this amazing modification for Unreal Tournament 2004. It's called Killing Floor.
You start out with a knife, a pistol and 4 clips, you've got 40 seconds to fortify yourself wherever you want, and then about a 100 zombies are released on the map. You've got to kill all the zombies while finding new ammo and welding doors shut to keep them out. You can heal yourself and your friends with med-syringes and buy stuff from a shop keeper that appears after each round (each 100 kills or so), you gain some cash every time you kill a zombie. Every round gets harder, new types of zombies.
The best thing about this game is the atmosphere. You have no cross-hair, so it's incredibly hard to hit them in the head, and there's a
filter on the screen, it looks really grainy, kind of like silent hill, so it really feels like a zombie movie. And, the sounds are really cool.