Here’s a project I’m working on, these days: libpuzzle, a library to find visually similar pictures
Using the library is easy. Give it a picture file (jpeg, gif, png), it will return a vector. The trick is that visually-similar pictures have very similar vectors, even if the picture have been resized or recompressed. And the library provides everything you need in order to compare these vectors.
So far, this is a C library. Ruby and PHP bindings will probably follow.
In the meantime, comments, suggestions and bug reports are welcome!