- Wolfram Alpha Flight Assistant
- Google Flights
- The Explore tab lets you set an approximate destination and date and it'll send you cheap flights.
- Rome2Rio is a catchall. Covers all forms of transportation.
- Skyscanner. I was told this was the best site to find cheap flights, but in my experience it's comparable to Google.
- Matt's Flights. Not sure what this is, but must have heard about it somewhere.