Sauerbraten or Cube 2 Sauerbraten is a free and open source first person shooter game for Windows, Linux and macOS. It comes with multiplayer & singleplayer game play. This is the successor of the old Cube FPS game. It is released under ZLIB license. It also comes with build-in map editor. This comes with some high quality graphics but you can play little older systems without any problem.
OpenArena is a free and open source first person shooter game for Windows, Linux and MacOS. The game is heavily based on the Quake III. It is Arena style deathmatch game. It comes with Deathmatch, Tournament, Capture the Flag, Overload, Harvester, Elimination, Last Man Standing and Domination game modes. In this game you need to score more frags to win the game. You can use guns, rocket launcher, shotgun, railgun and more in this game. It uses OpenGL 1.x graphics API and released under GPL-2.0+ license.
Red Eclipse is a free and open source single player and multiplayer first person arena shooter game built on Cube Engine 2 using SDL and OpenGL. It has six different game modes: Campaign, Deathmatch, Capture the Flag, Defend the Flag, Bomber and Time-Trial. It is available for Windows, Linux/BSD and Mac OSX.Red Eclipse takes influence from a variety of sources in its mechanics and gameplay. Flow, pacing, and level design are akin to Unreal Tournament, while aspects like burning, bleeding, and some weapon behavior have been adopted from Team Fortress 2. It comes with featuring parkour, impulse boosts, and more.
Xonotic is a free fast-paced first-person shooter that works on Windows, OS X and Linux.Xonotic is a direct successor of the Nexuiz project. The game is based off the Darkplaces game engine which is originally a quake modification. Xonotic has incredibly good graphics for an opensource game and is on par with most commercial games from 2006-2007. The gameplay is of course slightly inspired by several other games, but it is essentially a unique mixture of mostly original and different ideas.