A few nice search by image images I found:
TinEye
Image by schoschie
Extremely interesting new web-based visual image search engine that compares image contents (they claim to do a pixel-by-pixel compare, but I doubt that would be feasible computationally…)
[Update: Forgot to mention the important bit: instead of entering a search term, you upload or link to an image that you want searched on all of the web!]
[Update 2: I should try to find out the size limits of the image to be searched. What happens if I upload a 10 megapixel image? Can they handle it?]
tineye.com/
Seems to work pretty well, although the index is not very large as yet. I tried a couple of portraits of me and they weren't found :)
These are some interesting searches:
tineye.com/cool_searches
I wonder what they use to identify the image. I'm guessing they divide it up into blocks of pixels (eg. like JPEG's 8 x 8 px blocks) and create a hash for each, allowing to search just for parts of the image as well, which they do.
Interesting as well; searches are identified using a 40-hex-character hash, allowing for 16**40 different searches:
tineye.com/search/9f9d99705db1ca8fb9a73ac58b4732e5d859b687
