I love to travel & this site has me checking in on all my expected travel dates for the next several months. It seems to include most of the airlines I would use, and has been accurate and fun to use.
Highly recommend not just for the price trend data, but for squeezing in those trips you do not *think* you will be able to afford next year.
Afxf about Hopper: Flights, Hotels & Cars, v3.0.0