OpenArena is a free and community-produced deathmatch FPS that is heavily based on iD Software's Quake III Arena. Open Arena has very fast-paced action gameplay. There are 12 multiplayer game modes to choose from like: Free for All, Team Deathmatch, Capture the Flag, Domination, and much more. The game is very light weight and has very minimal system requirements, meaning it will run on any Intel powered Mac.
Mac OS X 10.4 or higher
64MB of system memory
OpenGL supported video card with 16MB of video memory.
300MB of hard drive space