Highly developed games that used to be P2P:

Lord of the Rings Online, Lineage II, Star Trek Online, Everquest II, City of Heroes, Dungeon & Dragons Online, Hellgate

F2P games that are highly developed because they are old:

Runescape, FlyFF, Ragnarok, MapleStory, Silkroad Online, Perfect World, Mabinogi

F2P that are newer but still well developed:

Allods Online, Runes of Magic, Atlantica Online, Forsaken World, Vindictus