In 2006, spammers started in a big way to use image spam to try to push through all of their stuff. While this technique is still used today, it isn't quite as effective because spam filters caught up.
One technique that Microsoft developed is called Shingling. That's where the image is broken up into a series of smaller segments, called shingles. The noise is removed and hashes compared for those microsegments. Given two images, it was possible to compare if two images were more or less the same. Of course, they weren't exactly the same, they were slightly different but all spammers were doing was inserting random noise, or rotating the image or phase shifting it. By ignoring the noise one could compare and match two images.
Recently, I came across the Photosynth application from Microsoft Live Labs. This is an application where you can upload your pictures from a trip and it will attempt to create a panoramic shot of all the images. I would guess that some of the image shingling techniques are used by looking at things like edge detection. While I was in China, I knew about this application so I took a few pictures to test this out. I didn't completely succeed in getting all of the overlap to work, but some of it turned out all right. Below is a shot of Shanghai, China:
Anyhow, it's a cool application. From now on, for my future trips I shall take shots of some neat places with this app in mind.