World of Warcraft. It's RuneScape, the browser based title from developer/publisher Jagex that has five million active players. In 2008 a graphically improved version of the game, RuneScape HD, was launched and Jagex states in a new press release today that the majority of RuneScape players have moved onto the updated version.
Today during GDC, Jagex has announced plans to launch an all new MMO title for release sometime later this year. The code name of the game is MechScape but other than that no other details of the game have been announced. Jagex claims the new title "will appeal to both existing RuneScape players and an entirely new audience of online gamers."