I prefer Kayak Explore for flexible itineraries since it basically just shows you the best price everywhere and you can filter by price, time of year, flight length: https://www.kayak.com/explore/

EDIT: And here's a ridiculous itinerary it found for reference:

$286, LAX <-> Oslo Round Trip, December 2 - 6, Norwegian Airlines: https://www.kayak.com/flights/LAX-OSL/2016-12-02/2016-12-06/

Eh, I prefer Google's calendar search for flights. I also don't care which days I fly and they'll show you the cheapest day to buy tickets.

Hitlist (hitlistapp.com) is also designed specifically with flexibility in mind - we don't have every single route on every day, but we let you set an alert with whatever criteria you have in mind - from 'SFO to London on these days +/- 3 days' to 'a beach in southern Europe for 7-10 days between January 8th and 31st' - and can ping you when fares that match come up.

TIL there are no flights between BWI and Dallas in January.

Maybe it's load but as others have said it doesn't seem to be returning complete (or in my case, any) data.

just tried my monthly delta route for a M-F out and back. no results, changed week, no results. I likely wont be returning to the site.