Google Flights has gotten quite good — it allows me to track flights over time so I can get the best price at the right moment. It's not as full featured as some other tools (and it's kind of cramped in Google's search UI) but it's pretty solid all around.
Oskar von Hauske- Designer @ Google
Data, tons of it
Jason Rados- Product Owner
The clean UI and speed make searching, exploring and monitoring flight prices a breeze.