Warzone 2100 is a free, open-source real-time strategy game available for Linux, Windows and Mac. This game was originally closed-source, developed by Pumpkin Studios and published by Eidos Interactive in 1999, but in 2004 it was officially licensed under the GNU GPL.
The latest version is 2.3.4, released earlier this year, on August 12. Let me list some of its features:
– single player and multi player modes
– 3D graphics engine, allowing the use of a 3D camera and angle rotation
– tons of units and upgrades to choose from
– allows fully customisable units