I agree with KimRaye as well, although I have found travelocity (via Yahoo) to be pretty much right on the money. I do know for a fact that Expedia often mis-quotes availability for hotels, but I don't know if that applies to airline tickets too.
One of the best ways I have found to keep track of airfares is via My Yahoo. You can set up a free Yahoo ID, and then set up a free My Yahoo page. On it, is the option of having 'best fares' displayed on your page, and you set it up to check fares for whatever airports you want...I have mine set up to check fares from Boston, Manchester NH and Portland ME to Orlando (of course!), NYC, Washington DC, New Orleans, and Atlanta. Its very easy to follow, and updates VERY quickly. I have My Yahoo set as my homepage, but still check it several times a day to see if fares have changed (they update that frequently). You can also have news, local TV, local Movie stations, and a bunch of other stuff displayed on the page, set to your personal preferences.
When the airfares look good, I then check with the airlines directly via their websites. This is because not all airlines are registered with Travelocity/Expedia (SouthWest and PanAm immediately come to mind) and they often have better airfares than the bigger airlines (and are usually the ones that spur the airfare sales).
BK