Nexuiz GPL is a fun and free GPL deathmatch shooter released in 2005. It is a free game: source code is free software and data is free content.
Video of Nexuiz gameplay with all eyecandy on for a high-end computer. Nexuiz can work on low-end computers but also scale up very well to state-of-the-art computers. The autor is GreEn`mArine, and was released on this thread of the Alientrap forums. You can read more details there.