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.
One important difference with this offering is that an application review is required before you can take your site live with the Expedia API. From the launch requirements page I can tell that Expedia’s motivation is to protect the consumer, which seems prudent since purchasing a flight usually involves hundreds of dollars.
Expedia takes it a step further with best practices recommendations. The fact that Expedia requires application review, and has also put together a best practices guide shows that they have been working with third party developers for some time. Expedia has dotted all the I’s and crossed all the T’s, essentially giving more than any humble implementer could reasonably ask.
The resources page lists a code library, tutorials, list of Expedia Affiliate Network API developers, and an XML sandbox. This API seems to have its roots in SOAP, so if that’s where your experience lies, you’re in luck. Expedia’s API offering is one of just 9 booking APIs and 77 travel APIs.