Photo album management. Based on the wavelet transform, which is simply a sophisticated averaging method. The best visual image search application I’ve found so far. Produces meaningful results. Based on solid mathematics. In fact, there is a paper describing everything. Of course, it’s still not exactly what I would like to see. Example  from the site: an image of sunset on a beach returns bright headlights in a city. Experiment: an image of a fingerprint is not matched with its rotated copy. The last one simply confirms what is discussed in the paper - the reliability of matching quickly deteriorates under rotations, distortions, etc.