When Carley Roney and David Liu got married, they had a seat-of-the-pants celebration on a sweltering Washington rooftop. They never planned to go into the wedding business, but soon saw an opportunity in the market for a fresh approach to wedding planning. In 1996, they founded The Knot, a website with an irreverent attitude about "the big day." The Knot weathered the dot.com bust, a stock market meltdown, and eventually grew into the lifestyle brand XO Group, valued at $500 million. PLUS for our postscript "How You Built That," how Michael Dixon's business, Mobile Vinyl Recorders, uses portable record lathes to cut vinyl at parties, weddings, and music festivals.