LiveJournal

Pick a spacific interest, but not too spacific. A good idea might be to look through your list of lj interests, and find some interests that don't yet have a community associated with them, but have several interested users. Then start a community about that interest!

I don't but I think it would be fun!! The best way to get people to join is simply word of mouth! Just post on your own journal or a few communities that are similar or where you'd think you'd pick up members who are interested and go from there! Good luck - there are a lot of communities out there which are just neglicted and unloved because no one posts in them.