I love a good optical illusion, so should everyone. Apart from the sheer joy of having our minds warped and being able to sit unbelieving, there is something nice about thinking through graphics and visuals and how our expectations and perceptions effect them.
Anyhow, dive onto the thread on forgetomori.com which covers this illusion (the thread has the propper PNG version and more details):
Just in case you thought, bah - nothing special there. Note that the green and blue spirals are the same colour...
This is very much like the illusion at the bottom of the Magenta article (which is very neato as well) - which is perhaps my favourite illusion: