Wesnoth, SuperTuxKart, 0AD or OpenMW are widely known. Being a bit ill these days I went through the FSF game directory, searching for FLOSS games that I'm unaware of. Maybe this can become a collection-thread for more obscure, second line projects?
3dyfication of a old atari 2d game.
Survival 2d game, active development:
Neverball: Game of Skill, comparable to the Brio wood labyrinth game.
Pianobooster: Educational Guitar-Hero-Clone for Pianos, aiming to train real piano skills:
Battle for the solar system, narrative 2D-Space-shooter:
Simple grand strategy game. Development seems death, but description is promising.