Bolide Software Duplicate Image Finder. Free download of software to use on your own image collection to find duplicates.
Google Image: To identify an image that you already have, save an image to your desktop, then drag into the search bar. Google will attempt to match the image.
Image Forensics Search System. "Search a target image within another image or search images that are similar to a target image."
ImageNet: "An image database organized according to the WorldNet hierarchy in which each node of the hierarchy is depicted by hundreds and thousands of images."
Image Similarity Search blog entry. Click on the comments section to get a number of helpful suggestions from readers.
Tineye: "a reverse image search engine. It finds out where an image came from, how it is being used, if modified versions of the image exist, or if there is a higher resolution version." Can also be used to identify an image for which you have no information.