Lugaru
Lugaru is a third person fighting game, the first game by Wolfire Games. It launched as a Mac exclusive on January 28, 2005, but since has been ported to Windows and Linux. It was included in the first Humble Indie Bundle, and was released under the GPL on May 11, 2010 per Wolfire's promise should the bundle raise $1 million.
Open Source Version
The source code is available for download here. Some assets, licensed CC-BY-SA 3.0 are included. There is also another project on Google Code.