Here is a website I have recently (within the last couple of months) stumbled across.
Its a pretty cool site.
You can go in and type which airport you want to depart from. It will then search the internet for published and un-published airfare specials. It will also tell you what last minute deals there are if you are looking for a weekend trip.
For example, type in STL for St. Louis-Lambert Airport and it will show you all the deals out of Lambert Airport.