Steve Kaufer got the idea for Tripadvisor in 1998 after spending way too many hours online, trying to figure out if a resort in Mexico was really as good as its brochure. When he launched a travel guidance site a few years later, his business plan failed spectacularly because he was trying to partner with other websites, rather than engaging directly with travelers. But Steve eventually arrived at a winning formula: make Tripadvisor available to everyone, aggregate tons of information about hotels and attractions, encourage travelers to add their reviews, and earn a fee from travel companies whenever users clicked to their sites. As the company grew, Steve remained at the helm, leading it through a $210 million sale to IAC, followed by a multi-billion dollar IPO in 2011. Today, Tripadvisor gets over 400 million visitors a month; and Steve—who just stepped down after 22 years—is already thinking about his next business.