Amadeus is a travel technology company dedicated to integrate travel ecosystem technologies. The Amadeus Asynchronous Scala SDK allows for callback functions, and adds the Travel Innovation sandbox to existing applications in order to create and test new prototypes. All GET calls require an API Key query parameter in the URL.
As a DC-area resident, I've been one of many local developers saying that the DC Metro needs some sort of API. If anything, just to be able to build or integrate transit routes into existing apps. The wait appears over and the Washington Metropolitan Area Transit Authority now has an API.
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.
Airbnb has announced a number of updates related to its API. For developers of software that allows Airbnb hosts to create and manage their Airbnb listings, there is a new feature that makes it easier to merge existing listings and sync settings, as well as a new dashboard and test sandbox.