We're hard at work planning our upcoming live show, so we bring you this favorite from the last year: Airbnb. A chance encounter with a stranger gave Joe Gebbia an idea to help pay his rent. That idea grew into a company that now has more rooms than the biggest hotel chain in the world. PLUS in our postscript "How You Built That," we check back with Michael Vennitti of TP Foam, a company that came up with a way to squelch the smell of trash.