imgSeek
imgSeek is a photo collection manager and viewer with content-based search and many other features. The query is expressed either as a rough sketch painted by the user or as another image you supply (or an image in your collection).
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Query images similar to one in your collection by double-clicking on it's thumbnail. Group your photos by similarity for easy browsing. You may also have them clustered automatically by color, date (group events automatically using an adaptive clustering algorithm for time differences), filename or image features.
I wonder what you could pull out of The Mirror Project with this.
It's a Python widget so in theory it shouldn't be too much work to add non-GUI interfaces or support for things RDF-ish . Neat.
See also : Perl Code and the Cosmic Radiation Background, parts one and two .