56. Luis von Ahn

Founder, reCaptcha

After helping to invent the now ubiquitous Captcha--the security word puzzles that virtually every Web site requires you to retype in order to weed out bots from humans--Carnegie Mellon computer science professor Luis von Ahn, 30, wanted to give the critical but mindless task greater purpose. "I felt bad," says the MacArthur Foundation genius grant winner and Microsoft New Faculty fellow. "They're not only annoying, but they kill 10 seconds of your time." Thus was born reCaptcha, which uses scanned print pages that are hard for computers to read as the word puzzles. Web users then unknowingly clean up the texts when logging into sites. So far, reCaptcha has translated decades' worth of New York Times archives, and in September, it was acquired by Google to help digitize all public-domain books.

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