Ourworld is pretty awesome, but if you are specifically looking for 3D avatars then I suggest you check out Meez. I play it a lot and so do most of my friends and it's pretty cool.
Lots. I'll give you some friendly ones.
-BinWeevils
-ClubPenguin
-AnimalJam
-Meez
-Woozworld
-Ourworld
And there are lots more but I don't want to list 100 sorry.