Stewart Butterfield, 32

In February 2004, Stewart Butterfield and his coworkers at Ludicorp, then engaged in developing an online game, launched a side product called Flickr -- "kind of on a lark." By summer, the project had taken over the company; today its the Webs fastest-growing photo-sharing site. Employing "tags" that allow people to make their photos searchable by content, Flickr encourages users to engage in discussions about their pictures. Acquired by Yahoo in March, Flickr now has more than one million users, who post hundreds of thousands of new photos a day.