Killing Floor is on sale today for the low price of $5! Pick it up while its so cheap, this is one of WaR's flagship games. It is a roundbase team zombie survival fps. Basically they put you in a map, there's places to hide doors to weld, etc and wave after wave of zombies attack and you have to fend them off as a team (6 players normally but there are other servers with higher caps). There are tons of weapons - flamethrowers, grenade launchers, katanas, etc
Again I highly suggest you get this one, we do a bunch of fun and funny stuff in it (for example 2 nights ago we all got rifles and lined up like civilized british redcoats and fought the zombies like gentlemen in a lineeventually we got eaten.
People who have this: Myself, Jedi, Cecil, TheFinisher, Wick, Knight, CJ_Guns, ThuGG, bPORR, Zaifos, AmbulanceParty
Seriously, just get it ($5) - http://store.steampowered.com/app/1250/