Don't forget FreedroidRPG, Pingus, SuperTuxKart, Neverball not to mention countless FPSs like Sauerbraten, Nexuiz (also in TFA) and a million clones of iD software games.

And that's only open source and freely redistributable games - on the commercial side there was also iD, Loki Games, LGP as well as a couple of indies even before Steam for Linux.