Sorry, I have to disagree:
Believe it or not, there are very good games in the free software world and Battle For Westnoth, Megaglest, The Mana World, Colobot Gold, Nethack, Openarena, The Dark Mod, Freedoom, 0.A.D, Sthendal, Powermanga, Redeclipse and Aleph One are examples of good free as in freedom games (At least the source code).
