A San Francisco startup hopes to turn ordinary Web browsing into a massively multi­player online game. In what Gamelayers calls PMOG (for "passively multiplayer online game"), players devise and follow "missions" that wind across websites (invisibly to nonplayers), leaving messages and prizes for one another. To get started, players download software that adds a toolbar to their Web browsers. When they log in to PMOG, software tracks their Web usage and gives them points for each top-level domain they visit within a 24-hour period. Those points buy tools that players can use to build missions, which can take many forms: a PMOG player might, for example, put a popup on the Boston Red Sox home page, inviting fellow players on a mission to learn about Red Sox history. At each site on the tour, a player following the mission would find a narrative written by the creator.

Along the way, players can send instant messages and links, leave gifts, and even plant little bombs that cause browser windows to temporarily (and harmlessly) shrink. "It's like instant messaging meets [social-­bookmarking site] del.icio.us meets Wikipedia," says company investor Joichi Ito, a board member of the Mozilla Foundation and CEO of the venture capital firm Neoteny. Ito believes that Gamelayers will draw participants who grew up playing video games; as players devise new types of games and game components, Ito says, advertising strate­gies can evolve accordingly. ­Justin Hall, ­Gamelayers' CEO and cofounder, says advertisers could create missions that incorporate advertising messages: Warner Brothers, say, might promote the next Batman movie with a tour of the superhero's history.