In a bold (desperate?) move the guys behind the nearly completely FOSS but still commercial MMORPG Ryzom have decided to make their game free-to-play, but with some smaller limitations (which are supposed to make you buy a subscription at some later point of course ;) ).
We mentioned it before, but Ryzom also runs fine under Linux (as seen above), and check out their developers page, and art asset repository if you are thinking about contributing.
Personally I feel the game shows it's age a bit too much on the desktop platform, but a mobile version (lets say on Maemo and Android) could really shine and attract a lot of users. But now is your chance to try it out as long as you like ;)