Gun Strike Blood Killer 3D!

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.

Squad is a 50 vs 50 multiplayer first-person shooter that aims to capture combat realism through communication and teamplay, emphasizing both strong squad cohesion mechanics as well as larger scale coordination, tactics and planning. It features large open maps, vehicle-based combined arms gameplay, and player-constructed bases to create a heart-thumping, visceral gaming experience which mixes organized tactical multi-squad planning with split second decision-making in real-world scale firefights. »

A free-and-open-source first-person-shooter aimed at improving the classic game, AssaultCube!

TrueCombat: Elite is a FREE modification for the Enemy Territory game (also free). It is a realistic modern day FPS featuring state of the art models and textures.

Nexuiz was originally developed by Lee Vermeulen and Forest ‘LordHavoc’ Hale, who started Alientrap in the summer of 2002. The goal of the project was to create a high quality first person shooter that could be played freely across all platforms in one package: PC, Mac, and Linux. The engine that powered Nexuiz was Forest Hale’s Darkplaces engine.

Wolfenstein: Enemy Territory is a free multiplayer first-person shooter. Set during World War II and heavily focused on team work, it was initially released in 2003 by Splash Damage and id Software. More than a decade after its release, after spending countless hours of escorting tanks and trucks, stealing gold and radar parts and transmitting confidential documents, it was time to dust off the game which consistently refused to go down.