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World of Warcraft (Activision-Blizzard)

Subscribers: 11.5 million

While “WoW” wasn’t the first massively multiplayer game, it took the category to new heights. Players pay a monthly fee of roughly $15 to play. And Blizzard keeps ‘em hooked with semi-regular expansion packs, the newest of which is due later this year.

It’s not the largest in terms of subscribers, but it’s by far the biggest in terms of revenue.

Photo Courtesy: Activision-Blizzard