Idee is a company that provides image search software, and they have a number of products including TinEye, which is a firefox plugin to provide query by example search for images. They provide a number of examples on various data sets (Alamy and flickr):
1. Search by colour on Alamy or flickr sets.
2. Visual search by example using an image from the Alamy set.
3. Visual search by example, using an uploaded image, either directly or using a URL on the Alamy set.
This is rather fun, enjoy! They have a blog.
Related: Hermitage museum provides similar search on their art collection, by colour and by layout, which are both great fun to use! Both provide demo's. This particular system is based on IBM's QBIC system.
(h/t: David Thompson)