Occupation: Co-founder, Twitter; co-founder, CEO, Square
Dorsey has not one but two billion-dollar start-ups under his belt. Twitter, of course, is the 140-character microblogging service that has already had a profound impact on media and pop culture. His new venture, Square, is a mobile-payments service that allows anyone to accept credit cards using a smart phone. A product visionary who emphasizes simple, elegant design, Dorsey has been dubbed "the next Steve Jobs." No pressure.