It is poptropica.
nijit101
One is girlsgogames.com. Those are just games. Virtual worlds? I am not sure if this is correct, I hear habbo.com? Best I could do sorry.
roblox
Xivio.com is great! Its a 3D no download free virtual world!!!
wizard 101
I think IMVU could do it?
nikohill
Weeworld and meez.
The best virtual world for tweens to play on is www.ourworld.com. it is easy to get started on and entirely FREE! On most virtual worlds you need to purchase a membership but on ourworld you don't. Most of the items are available for ALL members. www.ourworld.com is the best website for a tween virtual world.
Try Xivio.com, its a free 3D virtual world! No download too!
well there one i know switchin it a wedsite with 4 virtual worlds
MoshiMonsters.com ClubPenguin.com