[maxerickson]

Kayak has additional data sources. I don't know the details, but the additional sources is clear enough. This lists a whole bunch:

http://www.tnooz.com/article/kayak-to-google-bring-it-on/

[jeffbr13]

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.)

[gavinballard]

I used to use Skyscanner only for intra-Europe flights, but it's quickly becoming my first and last stop when looking for cheap deals. Really fantastic service.

[chriogenix]

used skyscanner quite a bit in europe however it has good prices on flights globally.