[STEAM] Steam - Killing Floor - Free to play this weekend!
More Info: http://store.steampowered.com/app/1250/
It's also 25% off this weekend as well.
About the game:
Killing Floor is a Co-op Survival Horror FPS set in the devastated cities and countryside of England after a series of cloning experiments for the military goes horribly wrong. You and your friends are members of the military dropped into these locations with a simple mission: Survive long enough to cleanse the area of the failed experiments!
I've never played it or even heard of it, so can't comment on it.