Simply the best thing that’s happened to travel planning since the internets.
Go to the ITA Software website here: http://www.itasoftware.com/
Click “Search Airfares Using QPX”
Log in as Guest.
From there, perform whatever search you need.
The thing that differentiates this from every other flight search is the option, on the upper right, for a “Month Long Search”. This allows you to view the cheapest fares for a given route over an entire month, making it incredibly easy to tell if its cheaper to fly on a Tuesday or a Sunday, etc.
The only catch, if you can call it that, is that you cannot book flight through this service. You should be able to book a flight online or over the phone using the exact dates, flight times and codes shown. I have, twice now. It could hardly be simpler.