Killing Floor,
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
Multiplayer
Red Orchestra: Ostfront 41-45
Red Orchestra: Ostfront 41-45. Red Orchestra places you in the most realistic WWII first-person multi-player combat to date on the PC, allowing the player to fight through some of the most intense combat of the war.
Multiplayer
Garry's Mod
Garry's Mod is a physics sandbox. Unlike regular games there aren't any predefined aims or goals. We give you the tools and leave you to play.
You spawn objects and weld them together to create your own contraptions - whether that's a car, a rocket, a catapult or something that doesn't have a name yet - that's up to you.
If you're not too great at construction - don't worry! You can place a variety of characters in silly positions.
Singleplayer
Three games that'll probably run fine on any PC and are very, very good.