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)
Compare Hotel Prices
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.