One of my 2019 resolutions is to take better advantage of traveling when flights are cheap vs setting vacations around (largely arbitrary) dates!

I feel like my friends who approach travel from the standpoint of going when flights are cheapest get around way more than I do, and I'd like to change that in the new year. I.e. I'd rather see a greater number of places during their off-season than only one or two places during peak visiting time! What tools/websites do you use to track cheap flights (outside of Google flight alerts)??