There is a great sight [sic] for man-made optical illusions, Michael Bach, University of Freiburg, Germany; the image below is from the site...¡OJO!...that means 'beware' in Spanish - the word ojo i.e. without the exclamation marks, means 'eye' - which is quite appropriate because on the website there are a pair of eyes that follow your cursor around the screen...it's fun at first but after a bit I found it quite spooky!
On a similar theme, my favourite tongue-twister, which in my experience has never been successfully 'spoken', is very simple really: quickly say, ten times, the following words:
"Red leather, yellow leather"
...in fact forget the ten times, I doubt you'll manage more than 3!
P.S. All new R5L poster/bloggers' links will be 'up' soon.