If you haven't played with http://google.com/flights yet, give it a shot. Especially for Europe, put in your starting airport, leave your destination airport empty, put in your dates, then scroll around the map.
i.e.
et voila!
Those are the lowest prices, taxes and fees included. Many times, there is a huge price difference between cities, when you can take a short train ride (time permitting). For example, flying to Paris or Geneva when the 3 hour train ride is about $100 return.
Once you've picked an airport, show the flights, then show the price graph. Here, you can see whether coming a couple days before or after would save you money.
Warning: you can lose a couple hours exploring, so grab a glass of wine first.
NOTE: You can also let Google find the best price in the next 6 months if you don't have firm dates.
Oh wow, thanks for the tip! I'll definitely be checking this out. I have a trip for Montreal planned in early September and we have an Airbnb but no airfare yet. I usually use an app called Hopper for plane tickets but it's not as good as it used to be.
Drinking while making travel arrangements might be a really bad idea for me though. I could drain our account pretty quickly. One glass might be okay. ; )