OpenFrag - An Open Source FPS
OpenFrag was founded in May 2003 and aims to create an open source first person shooter with commercial grade quality. All participants are volunteers and use some of their spare time to help in the development process. The project was recently featured last month at the Finnish game exhibition XPlay 2006. Some of the project's textures have also been featured in the news at BlenderNation, it looks as if there is a promising future for these developers.
Since Cube is not further developed anymore you might want to check out it's successor Sauerbraten