TinEye is the first image search engine on the web to use image identification technology rather than keywords, metadata or watermarks. It is free to use for non-commercial searching.
TinEye regularly crawls the web for new images, and also accepts contributions of complete online image collections. To date, TinEye has indexed 1,894,458,529 images from the web to help you find what you're looking for. Below is an introductory video.