What are some free virtual worlds for teens

There is a game called "SecondLife" that is like a virtual world. You can create your own character skins and create your own outfits and add them into the game, or, you can buy skins and outfits people have already made. You pay for a house and such. Although you can fly, it's very realistic. You pick a first and Last name and such. It's a 3D game too.