This is one of the prettiest things i’ve seen on the web in quite a while:
The team from Idée Inc. use Flickr’s API to generate a large corpus of ‘interesting’ images. Then they filter based on colour to show similar images. Further, their technology ‘Piximilar’ finds images that look like an image that you select.
I’ve done some work in the past with Media Bin from Interwoven, and they had a similar ‘search by colour’ feature. It’s great for brand images, and the results get better and better as you add more images. That’s why it’s fantastic with a huge body of free images like Flickr.
All of my pictures on flickr are licensed under Creative Commons – not because of any ideological feelings (I don’t have a problem being paid for work I’ve done), but because I like using other peoples’ photos when I can, and that system only works if most people abide by the rules.