If you enjoy co-op survival game you better head over to the Humbler Store within the next two days to get your hands on Killing Floor completely free! The price is originally $20, so you picking it up for free is a good deal. All you need to do is to follow this link and sign up/log in with your Humble account, and add the free game to the cart. After it's added you will get a Steam key that you can use to get the game on Steam.
Killing Floor is rated "Overwhelmingly positive" on the Steam user review section, so you will most likely have fun with this game. The plot is based around a military experiment that has gone wrong, and you need to survive, while also clearing the area of the failed experiments. Up to six players can play together, but it is also possible to play alone if you don't want to party up. Anyway, it's completely free, so give it a go!