6. World of Warcraft
In the past seven years, World of Warcraft has amassed an astounding 11 million paid user accounts worldwide. The mythos, graphics, and quest system brought those players to the 3D online roleplaying game, but the rich virtual world kept them coming back to venture on combat-rich quests with friends, collect rare equipment and build the stats of their online alter egos. As history's most successful pay-to-play massively multiplayer online game, WoW has inspired a seemingly endless stream of imitators, competitors and pretenders to the throne. In that way, WoW is responsible for the creation of more online worlds than most of the worlds in this list.