From duarte.framos:
The one I am interested in presenting is a type of "wipe effect" overlap tool. It basically places both images to be compared perfectly overlapping each other and then with the mouse cursor you can control a "wipe line" that controls which image is visible. This gives very very fine degree of detail because you can see the equivalent parts of the image right on the same spot, making it very easy to spot minor differences without having to highlighting them which is unpractical for high frequency noise and pixel small differences.