Please take a look at the entries for this years 'Best illusion of the year contest'
VOTE HERE: http://illusionoftheyear.com/vote/
These are not your run of the mill optical illusions, they are backed by cognitive research and made by some extremely clever folks.
My good friend Dr. Mike Pickard has submitted the following illusion, please vote if you get a chance.
VOTE HERE: http://illusionoftheyear.com/vote/