GWallet

Entrepreneur: Gurbaksh Chahal, 27
Funding: $12.5 million from investors, including Adams Street Partners and Trinity Ventures

For a video interview with Chahal, click here.

At the age of 16, Gurbaksh Chahal—his friends call him "G"—dropped out of high school to work full time on his first company. He became a multimillionaire when he sold it two years later. At age 25, Chahal sold his next company, online advertising network BlueLithium, to Yahoo! for $300 million. Now he's running his third, gWallet, which lets Web users playing games on social networks such as Facebook watch video ads or fill out surveys to earn virtual "currency" that they can use inside the games.

Toughest decision: Who to hire during gWallet's first year. "It's all about the team," says founder and CEO Chahal. "The first five employees will dictate whether or not you have a company."

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