We recently discovered an impressive bit of internet technology, it is still in beta but working well. You may love this technology, or fear it, depending on your point of view. In essence, it is a search engine for images. This search engine allows you to search the entire internet for a specific photo or image, including minor modifications made to it like cropping, etc. Read the full text for the full story.
While there is more info availabe on their site, here is briefly how it works. You upload (or provide the url to) the image you want to find. Their engine reads the image, pixel by pixel, and creates sort of a mathematical "signature". It then compares your image's signature against its database and provides a list of the matches. Somehow, it even detects matches when some of the pixels have changed (edited in Photoshop) or if some of the pixels are gone (cropped or resized).
Their website has a nice video with more details...it's worth a quick look.