itwixie - it's a site for girls 9-15 (but takes any age.) It's free unless you want to sign-up for chat, and has a moderator who checks posts before they go out. It's like a mini-facebook for girls.
webkinz- you have to buy the toy 1st then register basically you take care of a pet
neopets - like webkinz but you don't have to buy a toy
moshimonsters - adopt a monster and take care of it
girlgamesclub - a website for girls to play games. (The Sami games are the best)
allykatzz - a blog website with some games. Mostly blogs and gossip
mydinos - a place where you battle other dinos and explore the world
gogoddess - a website where you can play games and meet virtual godesses
chatandmorewebs- a website for tweens to chat, discuss forums, play games, look at photos, and more
girlsense - a place to create a boutique and sell your designs
avenue7 - a website similar to girlsense
britchicks - a place for girls to go on "adventures" and travel the worlds
toontown - a website to battle "cogs" with pranks and do toon tasks