[smcguinness]

Take a look at https://www.skyscanner.com. They have a "cheapest month" feature. All you have to do is choose your destination.

To see it, choose your flight then in the date pop-up choose Whole Month -> Cheapest Month.

I believe they also have the inverse where you can choose a month and a departure city and they'll give you destinations by price. I couldn't find it quickly though but I think they have it.

[zip1234]

While it doesn't work amazing everywhere, I love skycanner's map feature: Pick a month and destinations and prices shown on a map. https://www.skyscanner.com/cheap-flights-map

[Scoundreller]

I sometimes do the opposite for this: put my destination as my starting point, and use that to figure out which city is the best for me to fly out of.

I live near a border, and can arrange my life around a few other cities if the savings are worth it (e.g. visit family or see another city a few hours from me)