Back when Brian Chesky was an undergrad at the Rhode Island School of Design, his buddy Joe Gebbia made him pledge that one day they'd start a business together. "Sure, whatever," Chesky thought. Years passed. Chesky moved to Los Angeles to work at a design firm. Gebbia landed in San Francisco to work for Chronicle Books. Again, years passed.

"One day Joe calls me up and says I have a room available," Chesky says. "I impulsively quit my job and drove up there with a foam mattress in the back of my car to move in." He didn't realize until arriving that he couldn't afford to pay rent. That weekend, an international design conference was held in San Francisco. "I remember seeing on their website that all the hotels were sold out," Chesky says. "So we got the idea, Why don't we make our apartment into a little bed and breakfast?"  They hosted three visitors that weekend, and made more than $1,000. They loved having guests to show around, and, as a plus, they didn't end up homeless.

Another plus: People started buzzing about it. Conference attendees were saying they'd love a less expensive, more human-feeling place to stay. So the guys, with help from Nathan Blecharczyk, a Harvard grad who had worked at Microsoft, started a website, set on helping conference attendees find accommodations via listings posted by people with an extra room or couch. They promoted the idea on blogs and by telling "everyone we knew," Chesky said. "The idea is that you can book space anywhere. It can be anything, and it really is anything from a tent to a castle." SxSW 2008 was their first success - and then the Democratic National Convention came around. When Denver hotels sold out, nearly 1,000 listings on helped fill the lodging void.

Two years and a website overhaul later, AirBnB is used in nearly 5,000 cities in 142 countries. The company landed a $20,000 investment from Y Combinator; recently, the Wall Street Journal reported an investment from Sequoia Capital, but the trio of founders won't comment on that. The company is hiring so fast - and is still based out of the original South-of-Market apartment - that Chesky has been pushed out of his bedroom. That's right - he's pledged to be homeless. He's using only AirBnB to find accommodations for the year. A PR stunt to be sure, but also a test of the infrastructure of this laid-back but wildly popular business that started with a "yeah, whatever."