1. skyscanner.com
2. airfarewatchdog.com
3. hipmunk.com
4. kayak.com
5. skiplagged.com
6. matrix.itasoftware.com
7. qfly.com
8. farecompare.com
9. adioso.com
10. dohop.com
11. routehappy.com
12. momondo.com
13. hopper.com
14. google.com/flights
15. statravel.com
16. studentuniverse.com
17. fly.com