Warsow is a free, open source competitive FPS game built on Qfusion, a heavily modified version of the Quake 2 engine. In development for 7 years, the design is based on Quake 3 but adds new movement abilities – such as dashing, wall jumping, and ramp sliding – that are accessible via a special key. Additionally, Warsow has a number of features that make it easy to modify the game and spectate matches.
The game finally reached version 1.0 yesterday.
(Thanks, Türbo Bröther!)