Killing Floor

Killing Floor is a Co-op Survival FPS taking place 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 and police, dropped into these locations with a simple mission: survive long enough to cleanse the area of the failed experiments.
The only problem is, these "experiments" aren't waiting to be taken out - they´re coming for YOU!

Features:

• Cooperative gameplay for up to six players against multiple waves of specimens
• Persistent Perks system, allowing players to convert their in-game achievements into permanent improvements to their character's skills and abilities
• Slow-motion “ZEDtime” to better watch those crucial and violent creature deaths, even in multiplayer
• Solo game mode for offline play
• Nine different monster types trying to eat your face off, armed with everything from teeth and claws, to chainsaws, chain-guns and rocket-launchers
• 12+ weapons for the players to chose from, ranging from knives and fire-axes to pump shotguns, rifles and a flamethrower
• Equip your team with welders, medical tools and body armor to help your odds of survival
• Open, non-linear play areas: choose when and where to fight — or run; weld doors closed to direct the monster horde down alternate corridors
• Fully-configurable, allowing players to change the difficulty level, number of creature waves, or even set up their own favorite waves of monsters
• Support for Steam Friends and other Steamworks features
• Includes SDK for the creation of new levels and mods