Depends what you mean by 'MMO'. I really like how http://www.deadfrontier.com/ handles the multiplayers. Players set up servers, there's max 6 players that can play one level at time, but you have a character that lives in the world, thus you can get money, weapons & stuff by playing and then can trade it with others and join other servers using the stuff you have.
Works sweetly me thinks.
Works sweetly me thinks.