Urban Terror is a shooter game released in 2011 designed and developed by FrozenSand. Urban Terror can be described as a full free, open source game and can be played on the following operating systems: Windows, Linux, Mac OSX. Urban Terror is also a multiplayer game.
“Urban Terror™ is a free multiplayer first person shooter developed by FrozenSand, that will run on any Quake III Arena compatible engine. It is available for Windows, Linux and Macintosh.” - FrozenSand