September 26, 2003: I got all excited that there really was such a thing as visual diff, but then I realized it doesn't actually compare 2 images and tell you the differences like I wanted. It's just color-coded regular plain-ol' diff. And, yes, I know that what I want would probably be kinda hard.

Update: James pointed out that Photoshop's Difference blending mode does exactly what I was talking about. Thanks!