The overview includes only games which are completely free, with a permissive license for both the code and their content. I could not include here games like Urban Terror or Warsow, since their assets are not free, nor Steam games.
A very beautiful game, 0 A.D. is a 3D medieval strategy game with different nations, incorporating elements from various other games pertaining to the genre, like Age of Empires. Still in alpha stage (and has been for a long time), 0 A.D. is playable in both single-player and online. It has great graphics, specific units and buildings for each faction, and a selection of carefully designed maps.