As far as free legally...Priston Tale, Runescape, both are okay.

If you dont mind breaking the law, private Ragnarok Online servers can be pretty good. I dont even think it's really illegal for you to play on them, just illegal for the people hosting them(kinda like someone giving you a bunch of burned CDs for free.)

I'd really suggest playing a Pay to Play game though. There are TONS of good ones out there(I can't really say much about them though, only one I've played is WoW.)