This is one of the neatest optical illusions I've ever seen. Viewed up close, Mr Angry is on the left and Mrs Calm is on the right. If you back up, they swap places. Even works if you print it out. Link (thanks, suckup!)
Reader comment: Hello, Just wanted to send a followup to the Mr. Angry and Mrs. Calm optical illusion post on Boing Boing. My name is Michael Mack and I worked with Dr. Aude Oliva (she authored the paper about this illusion with Dr. Philippe Schyns) for a couple years at Michigan State University before she went to MIT (where she is now). I didn't do any work directly related to this particular paper, but we worked quite a bit on using this type of illusion for other psychological experiments in perception and vision. A slightly more famous example of this illusion deal with switching between Margaret Thatcher and Tony Blair (http://cvcl.mit.edu/gallery.htm). Dr. Oliva also has a with a couple more examples (http://cvcl.mit.edu/Papers/HybridFaces_AudeOliva.pdf).
The illusion works by manipulating the spatial frequencies of the image. The low spatial frequencies (the rough, fuzzy shape information of an image) from one image (e.g. Mr. Angy) are combined with the high spatial frequencies (the sharp edge information of an image) from a different image (e.g. Mrs. Calm). When viewing this hybrid image at a close distance, our perceptual system is able to extract and process the high spatial frequencies and thus we see the image where the high spatial frequencies were taken from. When viewing the hybrid image at a distance further away, our perceptual system can no longer extract the high spatial frequencies meaning we only see the low spatial frequency information, thus we see the other image. You can also get a switch in the hybrid image by squinting."