Check out:
http://www.onrpg.com/ for a massive list of Free MMORPG's.
My personal favorite is
Anarchy Online allthough I stopped playing the free edition and upgraded to Shadowlands three weeks ago...
I'm also following the developlemnt of
Planeshift. It's a long way from maturity yey, but it seems very promising allready and is more of less playable even in version 0.3!