http://www.truecombatelite.com/True Combat: Elite is a mod for Wolfenstein: Enemy Territory. Both of these games are free (but not open-source), cross-platform first-person shooters that use the id Tech 3 engine, much like everyone's favorite, Urban Terror.
Unfortunately, I have not yet had time to download this or try it out, but I intend to do so in the foreseeable future.
