Not sure whether there's a huge interest in it here, but I thought I'd mention that the source for the game Amulets & Armor was GPL'd on GitHub a while ago. Link to GitHub: https://github.com/ExiguusEntertainment Link to Wikipedia's article about the game: http://en.wikipedia.org/wiki/Amulets_%26_Armor The game itself is also free as in beer.
Speaking of source releases, Commander Keen's source was recently found, so hopefully we'll see it GPL'd too whenever id Software approves of it. Wikipedia link: http://en.wikipedia.org/wiki/Commander_Keen