I find optical illusions fascinating. You've presumably seen and no longer take delight in the "which bar is longer" or Escher'esque illusions, but those are the tip of an interesting and fun iceburg.
The above illusion is actually just a spinning grid, nothing fancy, but at certain speeds our brain will break up the large grid into several smaller, independently spinning ones. Click on the moving image to see some more examples and a possible explanation. It was discovered by David OReilly who has some other neat stuff, including an iPhone app called iHologram.
You may have seen some optical illusions that have to do with staring at one image for 60 seconds and then staring at another, where you see after-images remaining from the first image for a handful of seconds. This is due to fatigued receptors in your eye from staring at a saturated color too long. However, there is another similar experiment you can try called the McCollough Effect which has a much longer duration, as much as days after lookint at the saturated image. Interestingly, it is still a subject of research and nobody really can explain it.
Some optical illusions are exploited in software. The most common is the sense of the third dimension by using shading cues or converging parallel lines. Continuity and motion blur is exploited to trick you into believing an animation is smooth and not moving. I wonder though if we have barely scratched the surface in this space. If we understood better how the brain can be visually "tricked", are there new and interesting software interfaces that could exploit this fact?