Reverse image search
TinEye is a handy reverse image search engine. If finds where on the web an image comes from. You can use it to find where a photo of yours appears elsewhere, to find a higher res version of an image, or to locate the origins of a photo someone forwarded to you.
It does not use keywords, watermarks, or inbound links (as Google does) to locate images; rather it locates images via matching digital fingerprints of the image’s pixel arrays. This means it can find images that have been renamed, or cropped slightly, or even screen grabbed.
Currently TinEye is not exhaustive. In my experience it won’t find all the copies of an image. (They only claim to recognize a billion images so far, which is a small subset of all images on the web.) But it will find enough to be useful.
It can browse your hard disk for a target image, but even cooler is the Firefox plugin which enables you to select in image on a web page and in a click find where else on the web this image also appears.
(TinEye's business model seems to be using this technology in mobile phones to identify products. You snap a picture of an item and it gives you info about where to buy it, sample it, etc. Their first product (which I have not tried) is an iPhone App that recognizes CD covers (the few still being bought!) to provide you with the iTunes album options on your phone. If you have you used this App or anything similar and can report positively or negatively, please let us know below in the comments or via the submit page. -- KK — editors)