Kayak

Kayak is a travel metasearch engine that compares flights, hotels, rental cars, and flight-plus-hotel packages by searching a wide set of booking sites at once, covering destinations across many countries and languages, including one-way and round-trip flight searches and city-specific hotel and rental-car listings. It reports 41,000,000+ searches performed in a recent week, with results drawn from app ratings left by travelers.

Ad formats include Leaderboard, Medium rectangle and Wide skyscraper ads, all priced by impression auction, in US dollars. Its ads.txt file lists 28 ad system lines, 14 direct and 22 reseller.