Some of the cheapest fares recently have been the "unpublished fares". These are the internet fares available on the airline website or e-mailed to you after signup on the airline website. They are only short duration sales with travel time restrictions. Some of the airlines with these fares are US, DL, CO, NW and AA. However, trying to find "the cheapest" fare is like trying to catch a moving target, since they are changing as you read this. Isn't yield management wonderful?