The Raytex pinhole 'lens' is essentially a body cap style lens that mounts to the camera either directly or with an adapter. It is available in Canon, Nikon, Leica, K and m42 style mounts as well as some large format fittings.
It's creator Helmut Weber states:
"The Raytex PL42 is a precision engineered pinhole optic (lens) made of aluminium and stainless steel. The unique Raytex "Sandwiched Foil" system holds the foil perfectly flat and protected between two finely machined discs. The pinhole is laser cut with a diameter of 0.25mm - the optimal size to be used on a 35mm film or digital camera. Use f128 for correct exposure. All the Raytex Optics allow the use of extremely thin foils, even paper." [taken from an ebay listing]