Brand new, (one of the only few who has legal Japanese content) and just about to come out of Beta with incredible numbers:
Forbidden Tokyo
Let me know if you sign up, I will have to activate your account.
Besides that one, Payserve.com has a kick ass (and Legal) Japanese teen site.
Gotta watch out for Japanese sites, too many of them do not have the rights (or 2257 docs) on the content.