Urban Terror™ is a free-to-play multiplayer first person shooter game for for Windows, Linux and Mac OS X. It's a mod for Quake 3 Arena but it doesn't require it anymore, as it can run on ioquake3. Unfortunately Urban Terror is closed source, even though ioquake3 is under a GPL license, but that doesn't mean you can't play it.
Urban Terror can be described as a Hollywood tactical shooter; somewhat realism based, but the motto is "fun over realism". This results in a very unique, enjoyable and addictive game.
* The weapons available are from real life.
* The weapons have recoil.
* The weapons are less accurate when fired while moving (called movement penalty).