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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.
The Schiphol airport in Amsterdam has published a Flight API for developers to build new, creative passenger experiences. The API provides access to flight and airline data on the more than 90+ airlines that operate at the airport. Examples include flights, airline, status, destinations, and more.