Theres only a handful of games with true RP communities, and a lot of the ones mentioned above certainly arent them.

Vanguard as mentioned has a strong RP community, has since launch, and with the player built structures theres a strong sense of community in many places.

Everquest 2, Antonia Bayle has an amazing RP community, with many roleplay channels, many roleplayers, and a constant stream of tavern events etc, it takes the fantasy setting to a good depth, with good rp and maturity.

Ryzom, which im surprised hasnt come up yet, is one of the friendliest and most loyal RP communities you will find, its one of the few mmo's where you will find it harder to come across someone that doesnt roleplay, the game lives on RP.

SWG these days i dont feel offers the RP quality it used to, RP can be found but need a solid guild to prosper, as far as Sci-FI universes go thought, you wont find much better to RP in until SWTOR or Earthrise i expect.

Champions Online, for super hero based RP's theres no better setting, its taken a lot of CoX's good RP'ers with its outstanding character creation tools. You can find RP easily within a few minutes whether in a Supergoup or not.

CoX, while hard to find open RP, with a good Supergroup its a large world with both good and dark sides to it, offering a lot of places to carry out storylines. Population isnt at a strong point though, the expansion in early 2010 will decide if its worth it for newer players or not and for vets to continue.

If your into the Furry side of life, Furcadia is a community that is purely roleplay, theres nothing else other than RP and World Creation, player made spaces are the main focus, and everyone single person in there is there for RP, theres no other content.

Second Life, the only place you can create the exact setting you want to RP in, theres a lot of RP communities already residing in there, stay away from the ones with combat systems, they tend to be scripted weapon fests instead of good roleplay.

Fallen Earth, the only Post Apocalyptic setting available as an MMO, and with the recent social patch, roleplayers are being given more and more tools work with, player made structures and now making there way into the game, and theres a lot of RP based guilds in there.