After Warzone 2100, I decided to take a look at MegaGlest too, another 3D open-source strategy game.
MegaGlest is based upon the original Glest engine, but offers a lot of new features and capabilities, extending the original Glest (which is rather poor in options in my opinion) to a whole new game, including support for graphical resolutions, new factions, tech trees, tilesets and maps. It is available for Linux and Windows and it’s licensed under the GPL v3, while the game data is licensed under another permissive license, Creative Commons Attribution-ShareAlike 3.0 Unported.