Of course the old favorites are cool, but this one is TOO COOL!:
http://www.ebaumsworld.com/castle-illusion.html
(this is one you have to stare at for 30 seconds... and I promise nothing will jump out at you)

Anyone got a link to some cool new ones other than the ones that have been around for years?

3-D sidewalk chalk art (http://users.skynet.be/J.Beever/pave.htm). Check out the two pics attached: The first is the 3-d version and the 2nd is how it was done.

That's cool
I like it

For those of you that know how to view "Magic Eye" pics (http://www.magiceye.com/faq_example.htm), here's a cool one: