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 while ago, Id Software released the source code for the Quake 3 engine. They did not, however, release all the maps and models under a free license, so at the time, you still had to buy the game to play it. No more! Thanks to the OpenArena project, there's a cross-platform Quake client with free models and maps.