Most of the people who use their Mac don't use it to play games (something logical), but everyone wants to see from time to time they play some games of some type of game, and if yours are shooting and against other people in a multiplayer plan, then stay tuned.
Urban Terror fulfills the premise of being a fun game, of being online multiplayer, of being cross-platform (Windows + Linux + Mac) and above it is free, so you can't ask for much more.
It is based on the graphics engine of Quake III, and is the classic FPS (first person shooter). Enjoy it!
Link | Urban terror
I'll take it!!