Cofounders, Twitter (Ages: 36 and 31)

Photograph by Kara Andrade/AFP/Getty Images

It has become the ultimate tween-age diversion, a microblog that broadcasts real-time SMS status updates (OMG, guys r so hot at the mall 2nite!) to everyone in your network. Since its launch in 2006, Twitter has signed up more than 3 million users and is poised to make IMing as relevant as Friendster. Beyond the millions of inane news feeds, the service has less frivolous applications. Companies like Sprint and Whole Foods use it to provide customer service, the Red Cross disseminated aid information with it during this past summer's Midwest floods, and Barack Obama's Twitter feed kept 100,000 supporters tuned in to campaign news. But the surest sign Evan Williams and Jack Dorsey have created the next big thing is the number of copycats, like Jaiku (bought by Google), identi.ca (an open-source version), and Yammer (a for-business-only version). "No one wants to miss out," says Chris Shipley, chairman of the Guidewire Group, which does financial analysis for the tech sector. "But the key is creating a really important source of information—that's the real value." You know, information like what clothes you're trying on right now.

19. ROBERT DOWNEY JR., ACTOR >>