My favorite attribute of this website is you can put in your home location and then choose anywhere and cheapest month to find the best deals all while creating an adventure. Skyscanner allows you to search your destination and it will search a ton of different airlines and give you the best deal. You can also set up price alerts for different flights to watch if they drop.
Momondo and Skyscanner have a lot of similarities and I love to use both of them. You can put in anywhere for your destination, as well as choose places based on your budget. Momondo also has a calendar showing you when the cheapest days are for your flights!