There are lots of websites that offer to help you find great travel deals, but if you have a flexible travel schedule, I really love the Matrix Airfare Search Tool.
You start by entering your departure and destination cities, a 30 day departure date range and the range of days you can stay at your destination.
Once you do, it gather all the latest flight information and displays the cost to fly on each day of the month.
What I like about the way the information is displayed is that you can quickly determine if staying an extra day or cutting your trip by one day can save you a bundle on airfare.
Sites like Kayak or Google Flights are powered by this site and I prefer the way the information is displayed over those other sites.
You can’t book airfare through the site, but spending a few minutes on it before deciding which airline to book through can save you lots of time.