Free Killing Floor

Killing Floor is provided via Steam key for Linux, Windows, and Mac. For key redemption, a free Steam account is required.
DESCRIPTION
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!
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
Over 170 Steam Achievements, including “Dignity for the dead” for killing 10 enemies feeding on dead teammates' corpses and “Hot Cross Fun” for finishing off 25 burning enemies with a Crossbow
Slow-motion “ZEDtime” to better watch those crucial and violent creature deaths, even in multiplayer
Solo game mode for offline play
Ten different monster types trying to eat your face off, armed with everything from teeth and claws, to chainsaws, chain-guns and rocket-launchers
33+ 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
Choose which Perks to play with to best balance out your co-op team against the horrors
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 Windows-only SDK for the creation of new levels and mods