Best flight fares & Predicting flight fares from historical data

FareCast
FareCompare
Kayak US

Best fares search and prediction and historical data.
- Only available for US flights
- Certain cities and dates

For the UK, use Kayak UK
or Amadeus

(though historical data of best prices don't seem to take into account preferences of flights, e.g. whether it's direct flight, and departure times, etc. So should probably be best used to predict peakness/season rather than best prices I can get with my requirements.)