A picture for each eye - really! At a distance of about 2ft from your monitor, cross your eyes until you see three images - the center image will appear in 3D. It's called stereoscopy (or stereo photography) and it's been around for years.

You're probably more familiar with the 3D images you have to wear those funky red and green glasses to view. This is the same principle. Each eye is receiving it's own perspective of the image (a picture for each eye), thus conveying the original 3D environment in which the image was captured.

This isn't great. I was just reading about it and went out and snapped a dozen pictures before it got too dark just to see how easy it was. As with everything, it's easy to do - but doing it well is a different story!