No. 4: Sam Altman

Age: 22

Location: Mountain View, Calif.

2006 revenue: undisclosed

Employees: 30

Year founded: 2005

Loopt software provides location-based functions for cell phones, allowing users to see where their friends are on a map. The software also sends text alerts when friends come in close proximity to one another and can search for nearby restaurants. It'll even tell you what spots your friends have tagged as favorites. Loopt's service is available on Boost Mobile now and is expected to be launched on Sprint Nextel (S, news, msgs) and other carriers within months. Altman says he wants to bring the Internet out of the home and into the wild. "The company's mission is to enhance, improve and make more of real-world interaction," he says.

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Source: Inc.com

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