eChalk Optical Illusions (http://www.echalk.co.uk/amusements/OpticalIllusions/illusions.htm)
72 Optical Illusions & Visual Phenomena (http://www.michaelbach.de/ot/)
(http://www.uihealthcare.com/depts/medmuseum/galleryexhibits/eyeofbeholder/images/36dalamation.gif)
(http://www.homestead.com/coachburke/files/vanity_bw.gif)
(http://www.coolopticalillusions.com/illusions/skulltable.jpg)
(http://www.johnshanks.co.uk/wp-includes/images/optical/30.jpg)
(http://www.ophtasurf.com/en/illusions/illusion_visages.jpg)
(http://www.designboom.com/history/inversionimages/21.jpg)
Way cool illusion! (http://break.com/index/cool_optical_illusion.html)
(http://www.diycalculator.com/imgs/illusion-turtles-sm.jpg)
(http://markbowers.org/rw/illusions/ambiguity/files/page35_4.jpg)
(http://www.pal8198.jp/users/hmitsudo/illusion/shade.gif)
(http://www.diycalculator.com/imgs/illusion-snakes-lg.jpg)
http://images.google.com/images?q=visual+illusions&svnum=10&um=1&hl=en&sa=N&imgsz=medium
It's hard to believe that the squares at A and B are the same shade of grey
(http://i940.photobucket.com/albums/ad246/Snagglefritz/optical_illusion_grey.jpg)
You will find an explanation and proof here:
http://persci.mit.edu/gallery/checkershadow (http://persci.mit.edu/gallery/checkershadow)
And even more proof here:
http://persci.mit.edu/gallery/checkershadow/more_evidence (http://persci.mit.edu/gallery/checkershadow/more_evidence)