Alien Arena

Alien Arena is a free, stand-alone first-person shooter computer game based on source code released by id Software. Begun by COR Entertainment in 2004, the game combines a 1950s-era sci-fi atmosphere with gameplay similar to the Quake, Doom, and Unreal Tournament series. Alien Arena focuses mainly on online multiplayer action, although it does contain single-player matches against bots. Source: The Developer Website.