3D photos
Stereoscopy
Stereo photos are two photos one for the left eye, one for the right.
When the left-eye photo is on the left of the pair, you can view the picture with a special viewer, or use the "wall-eye" method, in which you stare at a more distant point than the screen/print and then move your eyes to the image without allowing them to cross as they normally would. When the left-eye photo is on the right of the pair, you can use the "cross-eye" method (you can use this with the other orientation, but near and far will be reversed), in which you cross your eyes just enough so that the two images make a third in the middle on which you focus.
Stereo photos have been made for over a century. "Holmes cards," antique stereo prints, can be found at many antique stores. http://www.t-enami.org/
If you don't have a stereoscopic lens, you can take two photos, one from a left-eye perspective, one from a right, then view/print them side by side. If you're not careful to keep the same background in both photos, you may have to rotate or translate the photo so that every analogous point is on the same vertical level between the left and right shots. I print these using Apple's Preview, in "Contact Sheet" mode, adjusting one side until the photos match vertically; then I print both photos on one sheet using the "Print Selected Images..." command, and choosing a two-per-page layout.
There is a size limit on stereo prints, since one cannot spread ones eyes further apart than parallel. Special viewers use a prism to help mitigate this issue. I use viewers from Loreo, the same company that sells stereoscopic lenses for SLR cameras. http://www.loreo.com/
