Rentalio is helps you find for vacation rentals. It is mash-up of Vast.com API with ambition to extend it to other useful sources of information for its users.
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flightwise (formerly fboweb.com)
Google APIlity Library
Kayak Search AS3
APIs, mashups and code. Because the world's your programmable oyster.
Executive Editor, ProgrammableWeb. Author, Map Scripting 101. Lover, APIs.