Sup Fags, i hate 37%. (of who/what you'll never know)
Now that thats out of the way, Killing Floor is an awesome Co-Op Zombie horde shooting FPS with zombie bosses, playable classes, and an array of guns and melee items.
It's pretty easy to run as well, so go get it and play it *3*
Killing Floor 2 is out, but if you and some friends who dont pc game much wanna mess around on a zombie game its a great one, shooting is some of the best in chibis opinion, and its Free so you cant go wrong.
It's also got a really great community, Ton's of cool maps to play around in, i've horde mode'd zombies in Zelda, Pokemon, etc, basically any cool place you may or may not wanna kill zombies in is a mod map somewhere.
Heres a vid if you wanna see how it plays.
And now here's the link to le free-ness: