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.
We strive to deliver the best gaming experiences on the internet and on your mobile phone. Play thousands of free online games, get access to free mmorpg games, online rpg games, fun online flash games, and more.
We offer free flash games in many different genres: online shooting games, online puzzle games, online war games, free online car games, free online hidden object games and dozens more.
This is the best place on the web to play online games for free! No matter what game style you prefer, we've got it here.