Scribd

Entrepreneur: Trip Adler, 25
Funding: $13 million from Charles River Ventures, Redpoint Ventures, Y Combinator, and the Kinsey Hills Group; angel investors include Marc Andreessen

Chief Executive Trip Adler came up with the idea for Scribd when he was a Harvard student and wanted to find an easy way to publish academic papers online. In 2007 he launched the Web site for uploading documents to the Internet and letting others read and repost them. Scribd now hosts tens of millions of documents, including popular e-books that publishers including Simon & Schuster sell through the site's online store.

Toughest decision: Last year, Adler decided to press ahead with a plan to let publishers sell books through Scribd, at the expense of creating a mobile application. "We should have focused earlier on mobile and later on commerce," he says. "We haven't always prioritized correctly, but we're getting much better at it."

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