Digital Paint: Paintball 2 was originally a Quake2 mod. Now that Quake2 has been released under the GPL, Paintball2 has become a complete standalone game with an enhanced, open-sourced engine. Between the Quake2 physics and the one shot kills, it's an extremely fast paced game with numerous maps and gameplay modes. Gameplay modes include Elimination, CTF, 1-Flag CTF, Siege, Deathmatch, King of the Hill, and Pong. Some of the engine enhancements include reflective water and skeletal animation.
Full standalone version of Digital Paint: Paintball 2.0 Alpha Build 20 for Windows (Quake2 not required!)