Plot: Alien Arena is a free, stand-alone first-person shooter computer game. 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. Wikipedia
Plot: Tremulous is a free and open source asymmetric team-based first-person shooter with real-time strategy elements. Being a cross-platform development project the game is available for Windows, Linux, and Mac OS X. The game features two opposing teams: humans and aliens. Wikipedia
Plot: Cube 2: Sauerbraten is a cross-platform, Quake-like first-person shooter that runs on Microsoft Windows, Linux, FreeBSD, OpenBSD, and Mac OS X using OpenGL and SDL. The game features single-player and multiplayer gameplay and contains an in-game level editor. Wikipedia
Plot: Smokin' Guns is a first-person shooter video game. Smokin' Guns is intended to be a semi-realistic simulation of the American Old West's atmosphere. Gameplay as well as locations are inspired by Western movies, particularly those from the Spaghetti Western genre. Wikipedia
Plot: Nexuiz is a free first-person shooter video game developed and published by Alientrap. The game and its media are released under the GNU General Public License and uses the DarkPlaces engine, a modified Quake engine. A remake, also called Nexuiz, has been released for Steam and Xbox 360 using CryEngine...
Plot: Angels Fall First: Planetstorm is an online multiplayer first-person shooter by British studio Strangely Interactive, combining game-play elements of traditional squad-based shooters with space combat ... Wikipedia
Plot: Wolfenstein: Enemy Territory is a free and open-source multiplayer first-person shooter video game set during World War II. It was originally planned to be released as a commercial expansion pack to Return to Castle Wolfenstein and later as a standalone game. Wikipedia
Plot: AssaultCube, formerly ActionCube, is an open source first-person shooter video game, based on the Cube engine. It takes place in realistic environments, with fast, arcade gameplay. Although the main focus of AssaultCube is multiplayer online gaming, a single-player mode consists of computer-controlled...
Plot: VDrift is a cross-platform, free/open source driving simulation made with drift racing in mind. It is released under the GNU General Public License v3. It is currently available for Linux, FreeBSD, Mac OS X and Microsoft Windows. Wikipedia