B&H is selling a used Canon Super Telephoto 1200mm lens for $99,000.
As for image quality, even wide open it's quite lovely. Stopped down to f/8 and f/11 it's actually quite remarkable. How remarkable? From midtown Manhattan we were able to read the street signs on the corner of JFK Boulevard East and 43rd St. in Weehawkin New Jersey when viewing image files at pixel resolution.
The lens weighs 36 pounds and there are probably less than 20 of them in existence. (thx, rob)
Update: Carl Zeiss designed a 1700mm lens for a 6x6 medium format camera. It weighs 564 pounds! (thx, jonathan, who notes that it looks like a Star Trek shuttle)
Update: There's also a Nikon 1200-1700mm lens and a Nikon 2000mm lens. (thx, markus)
Update: Canon even made a 5000mm lens. (thx, sadat)
Update: Ok, last one and then we're on to telescopes. (I'm kidding...please do not send me links to telescopes.) The Panavision 300x HD Lens...2100mm. (thx, philip)