With google image search (images.google.com) you can search by image. Now we can search Google with an image instead of text too!
Either upload the image or paste the image url to let google search related images for you on the web!
To search by image instead of text on google image search, simply goto images.google.com and find a small camera sign just to the right end of the input box.
Clicking this icon will give you a popup where you can select either you want to search images by giving the url of another image or by uploading the image to google
Here you can either paste image url or upload an image to search by image on google.
An awesome feature which I truly needed couple of times in past!