Airbnb's CEO, Brian Chesky, hinted that Airbnb may soon open an API for third party integration. Chesky envisions the company shifting from a marketplace to a travel platform upon which third parties can plug into. While no specifics were given, Chesky suggested that lack of an API is a hole.
Travel reservation site Expedia starting working on its Expedia API back in 2009, according to the changelog. It has three APIs, covering hotels, flights and cars. Don't be shy. Expedia has been expecting you. The Expedia developer site has all the warmth and care of a fine bed and breakfast. They clearly want to invite you in, make you comfortable, and get your development project of the ground. Though they do have a couple hoops to jump through, as well.