Go to Bing.com look under the travel section. There is a fare indicator and shows history of the route and current prices among many airlines. I use Bing all the time when searching for airfare (then book directly through the airline I choose)
Orbitz.com shows a calendar of leave/return dates and the (average) airfare.
You can click on the date you want, and it will bring up the flight information.
You need to check "3 days before and after."
I use http://matrix.itasoftware.com/ for employees. It searches a whole month and let you plug in # of days for example, May 20 & the next 30 days, 5-7 nights. it searches all major airlines. Unfotunately not SW, Allegient, Spirit but we don't use them.
For flexible dates I don't think there is a better website than http://www.google.com/flights if you use the buttons to the right of the dates (the user interface isn't very friendly) you can check a ton of dates and lengths of stay very easily.