Free is a pretty good price. That’s an obvious lesson from Sony Online Entertainment ‘s Free Realms massively multiplayer online game, which the company announced today has now crossed 2 million registrants.

That’s a huge swelling of players since the game launched on April 28, less than a month ago. It shows that recession-hit North American gamers are finally coming out in droves in favor of the free-to-play business model pioneered in Asia, where players start playing for free and pay for virtual goods one at a time in micro-transactions.

John Smedley, president of Sony Online Entertainment in San Diego, said the game is scoring well with the intended audience of kids and families. About 75 precent of the players are under 17 and 46 percent are under 13. Just 12 days ago, Smedley, who gave the keynote speech at our recent GamesBeat 09 conference, announced that the game has signed up its first million players. Nearly a third of the players are female.

Kids have been slow to adopt MMOs in the past because they don’t have credit cards to pay for subscription fees. But they can play this game for free and can buy Station currency cards to get virtual goods at five major retail chains: Best Buy, Blockbuster, Rite Aid, 7-Eleven and Target.

In the game, players can customize their characters, adopt pets, explore, quest, battle monsters and play a wide array of mini-games. It also has a trading card game, social networking, and cash cards that kids can purchase at Target, Best Buy and 7-Eleven stores. You can play a ninja character, decorate your house, or grow vegetables and flowers. Players can get into the 3-D game world and play within minutes.

Sony has operated a bunch of MMO games since launching EverQuest as a subscription game in 1995. It will be interesting to see if the players drive the company toward a virtual goods model. Wired has called Free Realms a cross between Disney’ Club Penguin and Activision Blizzard’s World of Warcraft.