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.
Note: I'm joking, there was also Chromium BSU.