These two are pretty trippy, this is the first time I've seen something like this. Check it out.
The thumbnail came from here:
http://newopticalillusions.blogspot.com/2007/08/confusing-reading-illusion.html
http://newopticalillusions.blogspot.com/2007/08/confusing-reading-illusion.html
This is an update of a much older trick where you stare at an inverted-color image of an American Flag and count to some number, then stare at a white wall and see the ghost image.