My kids are hung up on Club Penguin. They love it. It's interactive. There are games to play. There are other people to interact with (to a certain extent).

My oldest is the one that plays it the most and she's seven. My youngest, of course, wants to do what her big sister is doing. As a result, I succumbed to pressure this morning and got her an account to the tune of 60 bucks for the year.