imgSeek

ImgSeek is a unique Open Source, cross-platform project that aims to become the ultimate tool for performing local image searches with extremely accurate results.

The primary function of this software is to organize and manage your personal photo collection. First of all, you need to create a new database file, where all your image info will be stored, and add files to that database (you can apply several criteria when adding them, like omitting from size, dimensions and file extensions).
Once you have some files added, you can start playing around with your database.
You can browse the files as originally located, or perform some basic queries to the database, like grouping by keywords, date and some other.
But the most remarkable feature is the "query by content": you can select one file, and let the program find more "similar" files, based on its contents. This is done thanks to a very complex algorithm called "multiresolution wavelet decomposition". If you remember how the image looks, you can draw a rough sketch and tell the program to find it. You will be very impressed.
Concerning the GUI and usability, the program offers a basic functionality and could be greatly enhanced, but I think it is a good start.