Canon EF 1200mm f/5.6L USM
While the 1700mm lens mentioned earlier is all fine and dandy if you've got a truck to mount it onto, some may prefer a more lightweight, nimble sollution. Weighing a mere 16.5 kilos and being only 83 centimeters long (without the bucket-like hood), this delicate little flower will nevertheless magnify faraway objects (or perhaps more relevant, faraway people) to a degree that will leave little to the imagination. To my knowledge, this is the longest focal length available to autofocus SLR cameras without using any extra magnifiers.
Due to its size, limited area of use and robust price tag, it has only been available from Canon built to order, and to date they have apparently produced fewer than 20 samples of this lens. The company recently announced that they would be slashing the 1200mm from their catalogue, so if you want one, better be quick about it.
The suggested price of the lens upon unveiling in 1993, converted to present day money puts it at apx. $120 000, or the cost of "a small sports car" which is the most common price comparison given for the lens.