Second Life http://www.secondlife.com
There http://www.there.com
Moove http://www.moove.com/
Gaia Online http://www.gaiaonline.com/
World of Oz http://oz.ongameport.com/
Habbo http://www.habbo.com/
Active Worlds http://www.activeworlds.com
Club Penguin http://www.clubpenguin.com
Whirled http://www.whirled.com
WoozWorld http://woozworld.com