40 under 40

29. Dennis Crowley

Crowley shook up social media with his location-based software that allows users to broadcast their whereabouts, "checking in" for all who care. A year and a half after launching, Foursquare has some 3.7 million users; in June the company got a $20 million injection from big-name investors like Union Square Ventures and Andreessen Horowitz (see No. 1). One challenge: Others, most notably a company called Facebook, are on to location-based services of their own.

Track record: Crowley previously founded Dodgeball, a similar service Google bought in 2005 and shut down in 2009.

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Last updated October 14 2010: 1:25 PM ET