It's a pretty popular game on Steam; I used to play it, but haven't recently. It is a pretty good game. The one caveat is that this is Killing Floor 1, and there is a Killing Floor 2
that many (most?) people have moved on to. That's on sale also for half off (14.99).
Update: It's a steam key, and must be redeemed by 7/1. I already have it on steam, so if someone wants the key (an established DCer is the only requirement), just let me know.