These are both for United States departures I believe, so it probably would not help so much if you are traveling international.
Airfarewatchdog.com -- Simply input your departure city and you'll get a list of destinations and their fares.
BestFares.com has a feature like this, but I am not sure if you have to be a member of the site to get these fares or what. Here it is anyways:
http://www.bestfares.com/SnoozeYouLose.php