The Comeback Kid
The Comeback Kid
Photographed on the shore of San Francisco Bay
Jack Dorsey, 34
Co-founder and executive chairman, ttwitter ; Co-founder and CEO, Square

The cerebral software architect was ousted from Twitter only to return three years later to help fix it -- while also helming Square, his mobile-payment startup now valued at more than $1 billion. "When I was a kid I dreamed of being a train engineer or a sailor, and sailing around the world," says the St. Louis native. "It's not practical. And it's not as far-reaching as the opportunity with Square and Twitter. This is my dream job."

Last updated October 24 2011: 2:32 PM ET
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