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). The searching algorithm makes use of multiresolution wavelet decomposition of the query and database images.
On the left screenshot you can see what it means to search with a rough sketch painted by the user while the second screenshots shows a conten-based-search, "show me all my sunset pictures".
If you have enough money to waste there is also a non-free
server version so you can use a web based client version with similar features. Demo on:
http://fs.imgseek.net/.