AssaultCube

AssaultCube is a FREE, multiplayer, first-person shooter game, based on the CUBE engine.

OpenArena

OpenArena is a community-produced deathmatch FPS based on GPL idTech3 technology

Quake III Arena

Quake III Arena is a 1999 first-person shooter game developed by id Software. The third installment of the Quake series, Arena differs from previous games by excluding a story-based single-player mode and focusing primarily on multiplayer gameplay.

Notes

The OA server uses custom resources, which are found here, they can be auto-downloaded from the game if so you wish.

Games