Skyscanner (http://www.skyscanner.net/), who are bigger here in Europe, do the same thing as Kayak, only more-so - they have close to a thousand separate data-sources which they can query for prices, and they fall back on ITA and Amadeus only when necessary, because those queries cost more. Tends to result in better prices for the end-user, because the level of competition between vendors is phenomenal.

(source: was a Skyscanner employee for a bit.)