Shooters

Alien Arena is a multiplayer three-dimensional shooter with a first-person of arena. A free and open source game

Red Eclipse is a multiplayer first-person shooter in the arena, similar to Cube 2: Sauerbraten, Quake III Arena or Unreal Tournament. A free and open source game

GZDoom is a classic first-person shooter engine for all classic Id games. A free and open source game

Unvanquished is a first-person shooter that includes elements of real-time strategy. A free and open source game

Crispy Doom is a free and open source port of Doom with enhanced-resolution, limit-removing, based on Chocolate Doom

Wolfenstein: Blade of Agony is a free-to-play first-person shooter similar to Wolfenstein 3D, Medal of Honor, and Call of Duty, but with a more dynamic game in the spirit of Doom