TinEye.com is an new reverse image search engine currently in private beta. Upload an image or give it a photo url, and it will tell you where the image appears on the web!
While most popular image search engines look for images based on keywords you suggest, TinEye attempts to find images similar to the one you uploaded or linked, cropped images and even photoshopped images of the original! TinEye is indexing millions of images and smartly matches the fingerprint of your original photo to the images in their database. You can integrate it in your browsing habits using Firefox too.
For example I paste this image url from some website into Tineye, and it will find there are so many duplicate copies of the same image online, edited or not edited, with detailed urls etc. so that you can locate them exactly on the web.
The you can easily also sort these images by date, size and the amount of image editing done!
TinEye is built by Idee and this video shows you how it works. Request an invite and you might just get one to try it out. . Update: Now it is open to all. Easily find who copied your blog images online.