An interesting variant of the vdiff technique usable by anyone who has sufficient control over the parallax of their eyeballs (e.g. those who can easily view random-dot stereograms), is to hold up two paper printouts and go cross-eyed to superimpose them. This invokes deep, fast, built-in image comparison wetware (the same machinery responsible for depth perception) and differences stand out almost immediately. This technique is good for finding edits in graphical images, or for comparing an image with a compressed version to spot artifacts.
From the on-line hacker Jargon File, version 4.4.6, 25 Oct 2003