Well, OP is a given.
Eve seems quite popular around these parts if you don't mind the pay to play. (sci-fi)
I just picked up Guild Wars, a non-pay to play MMOG (or however that goes) but haven't played it yet so can't give a review. Some of my freinds and co-workers are quite hooked on it though. (D&D style)
Oh, one of the eve-heads can hook you up with a 14 day trial to check it out before you decide to pay for it.