Stewart Butterfield was another one of Technology Review’s 2005 TR35, our annual selection of top technology innovators under the age of 35. In 2004, he and his team launched Flickr, now one of the Web’s most popular photo-sharing sites, largely because of its innovative use of “tags.” Recently, Butterfield talked with us about how Flickr’s astonishing tenfold growth–from about 400,000 users a year ago to more than 4 million today–has changed nearly everything about running the site.