Imagine the entire Web as a gigantic game to be played, one enormous virtual world inhabited by "vigilantes" and "pathmakers" who are rewarded for, say, going out of their way to check in on Perez Hilton—or for avoiding Google.

That's the idea behind P.M.O.G., a free, browser-based "passively multiplayer online game" in which players travel the Web to earn points, directed by an algorithm that suggests missions based on their surfing history.

The game has quietly attracted some of the smartest venture-capital money in the world, including the respected Japanese angel Joi Ito, an early backer of Twitter and Flickr. It has also attracted 25,000 players in just a few months.

The idea behind P.M.O.G.—as with Facebook, YouTube, and other iconic Web 2.0 companies—is to build a large audience with a free service, then figure out some way to sell the audience to advertisers.

Like Facebook and YouTube, P.M.O.G. has succeeded with the first part: attracting a crowd. But also like Facebook and YouTube, P.M.O.G. is having a hard time with the second part: making money.

It does have one intriguing idea, though: using the game to steer players to sponsors' websites—or away from their rivals'—for a fee.

"I don't think it's a stupid model to attract eyeballs," says Ito. "The hardest part is getting the eyeballs to you. That sounds trivial. But the internet makes all kind of transactions possible."

Once players are in the game, it tracks their every move, rewards them for playing well—and, ultimately, could encourage them to fulfill the wishes of sponsors.

Justin Hall created P.M.O.G. after realizing how much time he poured into the online game World of Warcraft as a graduate student at the University of Southern California.

He enjoyed the social interaction of the so-called massively multiplayer game, in which people around the world play against one another without ever meeting.

But he said he believed there had to be a game he could play as actively as his schedule permitted, but could also be running when he turned his attention elsewhere.

What he came up with—along with two co-founders, his wife Merci Victoria Grace and friend Duncan Gough—is a game that people can play while doing something else on the Web.

As suggested by the company's name, GameLayers, P.M.O.G.'s free software can be downloaded to sit as a "layer" over the player's Firefox Web browser. While going about one's ordinary online business—conducting a Google search, for example, or checking Weather.com—players earn points based on the sites they visit.