This may have been posted before, but I've recently learned some interesting features of Google Flight search (https://www.google.com/flights/). First, after you search and select your flight pairs, you can save the itinerary (it's an option beneath the "purchase" buttons). Later you can go back and see historical price changes graphed for those flights (they're stored in "Saved Flights" area using the menu on the top left of the search page). It starts tracking the day you save the flight. So very handy!
Another great tip is the map search option. You type your home airport, select travel dates, and then click on the map below the search area to explore the current prices to destinations world wide. If you select a destination city, it also shows a graph with prices near those travel dates. The main search function also lets you know if adjusting your travel dates a few days in either direction can save you money.
Lastly, if you know where you're going already, if you put in your city pair and click the calendar button next to flight dates, it shows fares in the calendar for all dates, not just the date you selected. Unfortunately this feature doesn't work for open jaw tickets, but it's handy for estimating.
I used to be a loyal Kayak user, but these features have changed my mind.