Digital Paint: Paintball 2!

Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux. The project is geared towards providing addictive arena shooter gameplay which is all spawned and driven by the community itself. Xonotic is a direct successor of Nexuiz with years of development between them, and it aims to become the best possible open-source FPS (first-person-shooter) of its kind.

Red Eclipse is a fun-filled new take on the first person arena shooter, built as a total conversion of Cube Engine 2, which lends itself toward a balanced gameplay, with a general theme of agility in a variety of environments.

Warsow is a completely free fast-paced first-person shooter (FPS) for Windows, Linux and Mac OS X, set in a futuristic cartoonish world where rocketlauncher-wielding pigs and lasergun-carrying cyberpunks roam the streets.

Cube 2: Sauerbraten is a free multiplayer/singleplayer first person shooter, built as a major redesign of the Cube FPS.

Urban Terror started as a realism based "total conversion" mod for Quake III Arena. Nowadays, Urban Terror is played using the free, stand-alone, ioquake3 engine modified, which doesn't require Quake III Arena, in addition to being an add-on to id Software's Quake III Arena.

Zandronum is a multiplayer oriented port, based off Skulltag, for Doom and Doom II by id Software.

Alien Swarm is a free top-down shoot-em-up video game by Valve Corporation. It is a remake of the Alien Swarm mod for Unreal Tournament 2004, and it was developed by the original team, which was hired by Valve during the course of the development process.