In a perfect world, I would love a site that searches all of the airline fares and mileage programs, including current bonus mileage promotions, and calculates the cents per ff mile earned cost of a particular trip.

A person could put in their departure city and perhaps their status level in their ff accounts (so that your program would know to double their flight miles for example if their ff program gives them double miles for their elite status) and your program could factor in current bonus promotions, and the cost of the ticket to arrive at the cost per ff mile earned. This would help for pure mileage runs as well as "normal" travel.