Bad Astronomy has a link to an awesome illusion. I recommend it... unless you are prone to visual epilepsy or possibly those who easily become motion sick. Not much else to say about it.... why are you still reading this? Go to the illusion.
Edit: I just showed it to a friend in the grad office, he said it was like a mild version of the illusions he had while he needed to use pain medication. After a day of it, he decided the pain was more tolerable. I can see why, the effect lasts less than a minute (and is thus fun), but if it went on a whole day it'd get old real quick.