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.
Or STK for short, is one of the most popular racing games available freely and open-source on Linux. You can find install it in any major distribution with the default package manager. SuperTuxKart comes with several game modes, like single racing or Story mode, vehicles models, inspired by mascots in the free software world (you can drive with Tux, FreeBSD devil, GNU etc) and a whole bunch of maps, which you can unlock by completing missions in the story mode. STK also includes weapons to slow down your opponents. There is an in-game add-on menu from which you can download even further maps and models.