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. On offer until 7 November. http://store.steampowered.com/app/1250/
Yeah I saw that. The game doesn't seem all that impressive, but does look fun. I do a lot of cooperative gaming in my LAN Parties. Maybe this is one I should pick up?
Unfortunately co op is only available online This means I have to get all of my friends to purchase it too in order for us to play. I run all of my LAN Parties offline with Steam and one account. Works really well, but so many games require you to be logged into Steam to play cooperative.