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Founder runner-up: Jack Dorsey
Creator, co-founder, and chairman, Twitter

What started out as a way for Dorsey to let friends know about his goings-on has gone from a side project to a wonder rivaling Facebook. Twitter lays claim to more than 75 million users who broadcast their thoughts in 140 characters or less. Now Dorsey, 33, is on to his next project, Square, a mobile-phone payment service that relies on the same tenets as Twitter -- immediacy, transparency, and approachability -- and enables users to conduct credit card transactions with a phone and a free add-on device in 10 seconds or less. -J.P.M.

NEXT: Smartest Engineer: Christophe Bisciglia
Last updated July 09 2010: 1:19 PM ET
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