Unvanquished!

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.

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.

Alien Arena is a free and opensource first-person shooter game that combines a 1950s-era sci-fi atmosphere with gameplay similar to Quake (series) , Doom (Series) and Unreal Tournament (series) . Alien Arena focuses mainly on online multiplayer action, although it does contain single-player matches against bots.

Tremulous is a free, open source game that blends a team based FPS with elements of an RTS. Players can choose from 2 unique races, aliens and humans. Players on both teams are able to build working structures in-game like an RTS. These structures provide many functions, the most important being spawning. The designated builders must ensure there are spawn structures or other players will not be able to rejoin the game after death. Other structures provide automated base defense (to some degree), healing functions and much more... »

Blacklight Retribution is a fast paced, adrenaline pumping free to play first person shooter (FPS) based in a futuristic urban warzone. Blacklight Retribution provides FPS enthusiasts with a massive arsenal of weaponry and gear ranging from bleeding edge side arms to massive mechanized hardsuits.