f.75

You might think you have a fast lens if it tops f/1.2 or f/1.0, but that's nothing compared to the De Oude Delft 50mm f/0.75. It's one of the fastest lenses in the world, and -- compared to other super speedy lenses -- isn't that expensive.

Flickr member scenery_and_fish found a Kowa 65mm f/0.75 x-ray lens, and mounted it to a Nikon D90 by using macro extension tubes and epoxy. The lens is fixed focus, lacks an iris, and is one beastly piece of glass.