I don't know about open world, but Fable is good. I think they've been mentioned already, but the Elder Scrolls games (Morrowind and Oblivion) and the Diablo's. NeverWinter Knights, Knights of the Old Republic, all of the Fallouts (1, 2, 3, and NV), Baldur's Gate. If you're into the paid subscription stuff, then of course nothing beats WoW (in terms of subscribers), but I like Everquest, Eve is a good space MMORPG. Those are just some I can think of off hand.