Another one of the
coolest illusions I have ever seen:
Download this program:
http://www.grand-illusions.com/Optical.exe
and run it (I just checked it for viruses, and found none - this has been on the 'net for years, btw). You will see something like this, except in a
moving sprial:

stare at it for about 20 seconds or more, and then look at your hand. If this doesn't freak you out, nothing will.
(Note: Staring at the still image will do nothing except waste your time. You have to run the program. Also, this was taken from this page:
http://www.grand-illusions.com/pinwheel.htm )