Some illusions I like...

http://www.skytopia.com/project/illusion/illusion.html

Give "The eclipse of Mars" a go. I got some weird looks for sitting nose pressed against the screen for over 2 min's with tears rolling down my face trying not to blink. Well worth it though.

 

I like the rotating snakes, and I've had the checker-shadow for years as it's a good-un....doesn't matter how you look at it those shades of grey look completely different, but I've checked it (as have many others I should think) on graphics progs and it's the same.

 

Yep. Seen many of them before and am guilty of having checked the checker-shadow one too. Did you try the eclipse of mars? First time I've seen that one.