There are many massive multi-player online games available. I’ve introduced some in the past and I think they represent a very interesting language use/learning option for many learners in Japan. They are usually not cheap, however, often in the range of 1500 yen per month. Recently though, several games have drastically lowered their fees or even made the games free to use. Here is a list of some games that might be good (and cheap) for language learners:

For Older Learners
EverQuest II Extended
Pirates of the Caribbean

For Younger Learners
Free Realms
Wizard 101