Back in 1999, a Real Time Stategy (RTS) game called Warzone 2100 was released by Pumpkin Studios. In 2004, Warzone 2100 was released as open source under the GNU General Public License (GPL), by its copyright holders Eidos-Interactive. A group of developers have created a project to maintain this source code and add new improvements, and have released multi-platform versions of the game (including a Linux version):
Updated 2021 : I am delighted to see that in 2021, this project is still going strong! Now I really need to download Warzone 2100, I remember having so much fun with this game back when it was originally released.