I'd say Neverwinter NIghts - has great RP servers and lots of modules. I haven't played (enough) NWN 2 to make a judgement but I'd assume it's the same case. There is no compelling reason for it not to be.
Second, if you like Vampire the Masquerade, VtM redemption gives you good tools to create and run your own RP multilayer game. I don't know if there are a lot of servers/people to play with, though.
Both of those aren't MMO, though, and they need a game master of some sort to run them.
Maybe there is an RP server or something in Dungeons and Dragons Online but I haven't investigated. It is D&D and a MMO, though, on top of it, it's free, so it might be worth looking into.