America's Army: Special Forces 2.0.0a

America's Army is a free online FPS game published by the U.S. Army, and it uses the Unreal graphics engine to create a remarkably authentic military simulation. Special Forces have been added to the game and they get to use new, highly specialized weaponry including a customizable M4 Carbine and a Thermite Grenade. To play as a Special Forces Soldier, you will need "a minimum HONOR of 15, completion of all the Special Forces training components, and completion of both Basic Combat Training and Airborne School training components." It's available for Mac and Linux as well as Windows.
Poll: Game Wars - Wolfenstein: ET vs. America's Army.