Free | Anti-Twin
Anti-Twin is a great tool that allows you to find duplicate images on your computer. It compares images with a customizable level of accuracy. This allows you to find duplicate images even if they are slightly different, such as quality differences between two images. The search compares images from either the same or separate folders and displays the findings in an easy to use formatted list.
The times I have found this program the most useful have been when I have mistakenly copied over the same project from one computer to another multiple times. Sometimes I will have duplicate project folders in different location on my computer, with Anti-Twin I am able to find those duplicates and remove them freeing up space.