Sony Online Entertainment’s Free Realms online game has pulled in 20 million registered users since its launch in the spring of 2009, the company said today.

That’s a pretty big milestone for a fantasy massively multiplayer online game. Sony Online Entertainment, the San Diego, Calif.-based division of Sony, has been able to do it because the title is free-to-play, meaning users can play for free and pay real money for virtual goods such as decorations or weapons.

Sony has never disclosed its monthly active players, which is a more telling feature, since it shows how many players enjoy the game on an ongoing basis rather than those who just tried it out once. The game is available on the PC, Mac, and PlayStation Network, and it features a bunch of minigames, adventures, and other game play activities. It is aimed at a wide audience, including teens.

John Smedley, president of Sony Online Entertianment, said that the benchmark is a testament to the company’s success with the free-to-play business model. Sony continues to refine the game so it can continue to attract new players. The company is offering a special promotional in-game varsity letterman jacket to each player who logs in from Oct. 21 to Oct. 31.