Six months ago, Google released an image search labs project that allowed you to search by similarity within image search. Google announced that they have promoted that labs project as a default feature in Google Image Search.
Now when you search on Google Images, you may often find a link under the image that says, “Find similar images.” When you click that link, Google will show you images that look similar to the image you were looking at.
Here is a picture of how that link looks:
I personally tried it this morning, but for some reason, non of the example searches work, nor do my own examples work. It is possible that this feature is not fully launched yet or that Google has pulled it due to a temporary bug. I am not sure, but if you do not see it, you can always play with it at similar-images.googlelabs.com.