4. Skiplagged
Although, they are under a lawsuit with United, they are still working fine to provide the best flight information and booking service for free. The unique hidden-city ticketing method provided by Skiplagged can help you book flights that can go up to 80% cheaper than any other sites in the business. It is upto you to decide how morally obligated are you to take advantage of the loophole that is present in the airline industry. Either way, this helps you book the cheapest flight possible using this particular app.