Gaming on a Mac is generally a bad idea and you're far better off getting yourself an equivalent PC for half the price. I just have to put that out there.
But if you absolutely must, Runescape and Second Life are free-to-play (after a fashion) and not terrible as far as I know. League of Legends isn't quite an MMO, actually it's an MOBA, but it's still a very popular free-to-play online game.