I found this by accident which means one of two things - It's not documented on any page I've used or I'm finding out something everyone else already knows. I bet it's the second but...
In Google image search when you hover over a thumbnail and it 'pops-up' a slightly larger version with details you can drag and drop that image back onto the search bar and a 'drop image here' message appears. Google will then display links to other sites with the same image as well as a similar images box.
Well done, and quite intuitive imo, I like it.