I used to enjoy a feature that existed years ago on Travelocity or Expedia (not sure which) that allowed you to type in a dollar amount and a date, and it would tell you where you could go for that amount.
I just today noticed that Kayak has a feature called Explore that opens a map, allows you to set your starting point, and tells you the lowest economy class airfares from that point have turned up in Kayak searches in the past 48 hours to all the cities on the map.
Unless you're looking just for fun, it won't be the most helpful tool because in that past 48 hours someone may have searched Paris for a low-season month while you're looking for August...but still...