+1 for Killing Floor.
Oh yes, killing floor is great fun. No PVP , but the co-op itself is as challenging as **** on higher difficulty settings. Also has a "bullet-time" feature when someone pulls off an awesome kill. Best thing about Killing Floor is the class progression. So unlike L4D/L4D2 , you don't have access to good weapons immediately. Killing zombies and earning experience will let you unlock them.
Lots of zombie games came in after L4D,L4D2 but Killing Floor is the only one that I've found decent so far. Don't play it any more though.