What's that you say? House of the Dead, Left 4 Dead, and CoD World At War don't quite scratch your zombie itch? We've got just the game for you. Killing Floor is a total conversion for Unreal Tournament 2004. Here's the description of the game from developer Tripwire's forums:
Tripwire Interactive is proud to announce Killing Floor, due for release over Steam in the near future. Killing Floor is a first-person co-op survival horror, set in and around London after a mad scientist’s “specimens” have broken loose and gone on the rampage.
Big Ben and braaaiiiiinsss? Sounds like a winner. In addition to offline single player, up to six players can co-op through the streets of London, blowing zombies away in unison. Killing Floor is coming to Steam "in the near future". We'll be sure to keep you posted!