[link|http://www.versiontracker.com/dyn/moreinfo/macosx/23459|SuperTux] - Mario Brothers clone with Tux as the hero

[link|http://www.versiontracker.com/dyn/moreinfo/macosx/18406|Frozen Bubble ] - Very addictive, puzzle/strategy game

[link|http://www.versiontracker.com/dyn/moreinfo/macosx/17608| BZFlag ] - It's capture-the-flag with tanks. No single-player mode, unfortunately. Multiplayer over the network/internet.

[link|http://sourceforge.net/project/showfiles.php?group_id=99318| Cube ] - Actually an open-source first-person shooter game engine, but you can still play around with it. Nifty graphics.

[link|http://source.bungie.org/| AlephOne ] - Bungie released the code for their innovative (for its time) FPS Marathon to the open source community. This is one of the most notable results. It's designed to be installed with a licensed copy of Marathon 2 or Marathon Infinity, but it works fine with the freely downloadable demos of these.

[link|http://legacy.newdoom.com/| Doom Legacy ] - Even more "old school" but in the tradition of AlephOne, here's what happens when the original Doom gets released to the open source community. I remember this as being one of the creepiest games I ever played at the time.