Great thread.....there are some sites listed that I think my kids would enjoy. My 10 year old daughter has a paid membership to Star Dolls and my other daughter has a paid membership to Pixie Hollow. They also like yahoo kids, disney, dragon fable and adventure quest.
My 9 yr old, his friends, and 50% of the boys in my class LOVE Roblox. My DS is getting addicted, so I have to limit his time. He likes playing it online live with his friends. They talk on speaker phone while playing the game together. There's a free version, but it costs extra be part of the Builder's Club where players ear "Robux" that they can buy things with. The site also has lots of fun contests. http://www.roblox.com/
If your DS likes Legos (and even if he doesn't) he'll enjoy this site. It's safe and moderated.