Tripwire’s original Killing Floor is free at the Humble Store today, and tomorrow. The co-op zombie exterminating game can be picked up for no money at all until around 10am (Pacific) on 24 June. You’ll need a (also free) Humble account to do so.
Like Left4Dead it’s not a whole lot of fun solo, but if you can encourage a few friends to get a free copy too then you can get together for some zombie-gibbing laughs. Or you could try playing with random people, since this giveaway will likely expand the player base.
Here’s an old video showing what Killing Floor looks like, in case you’re unfamiliar. In short, you need to prevent horribly infected monstrosities clawing you to death by any means necessary.