EQ2 and Vanguard are the only ones, and frankly the amount of change they allowed characters I thought was stupid; it never made any sense. How on earth can you change your skin tone, height, entire facial structure and so on at a whim?
Although I don't actually play any of these new-fangled MMOG's, my guess would be that in something like Second Life, EVE or Anarchy online, they would use 'science' performed by a plastic surgeon, bio-mechanic, genetic specialist, etc.
In the fantasy games, such as World of Warcraft, Lord of the Rings or Age of Conan I suspect they would use 'magic' as used by wizards, magic-users, necromancers, shamans, Stygian priests, etc.
But I could be wrong. Perhaps the idea of changing your looks for cash is a really far fetched idea.