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Similar Images is graduating from Google Labs and becoming a permanent feature in Google Images. You can try it out by clicking on "Find similar images" below the most popular images in our search results.
Similar Images from Google Labs
It worked both as a disambiguation tool and as a pattern analysis tool... and the pattern analysis could be skewed by color. Isolating the search (via photographer's name) to black and white images with no defined subject gave a better idea of how the pattern analysis worked....
The search considers a combination of meta data and image analysis. Results are amazingly good, but not perfect.
To check out the image analysis by itself, I tried searching by the name of a former Life magazine photographer whose work is all in black and white, and areas of light and dark were initially big factors in what similar images were returned. I could keep refining the search, though, just with the "similar" link, until Similar Images zeroed in on multiple copies of the same photo.