I have had a pet interest in the 3D photography technology called stereoscopy ever since my mom gave me a stereoscope of 19th century design for some boyhood birthday. Although the technology remains a small niche of photography, it is being adapted to the digital age.
I recently came across the Loreo 3D Lens in a Cap, a stereo lens that works on most film or digital SLRs. It's a 38mm lens with an f/11 of f/22 aperture that takes two images of the same scene from slightly different perspectives.
With stereoscopy, your brain can reconstruct depth information from the two images, just as it does with the two views from your eyes.
Loreo also sells a basic $24 3D viewer into which you can put 4x6 printouts of your stereo photos. The lenses cost between $48 and $117, depending on what model you buy.
(Via Red Ferret.)