Skyscanner API

Flight, car hire and hotel meta search (price comparison) API

FlightStats APIs
Everything about flights, airports, routes and schedules

Uber API
Integrate Uber in your application

Rome2rio API
Multi-modal (flight + bus + train etc.) travel search

Wego Affiliate Network
Meta search for flights and hotels

Obviously there are also the APIs of the 3 major GDS’ Sabre, Amadeus and Travelport.