My eyes, my eyes! These are just the tip of the iceberg, from the excellent site by Michael Bach - http://www.michaelbach.de/ot/index.html
"Stare at the centre of the figure for a while. Some ‘scintillating’ activity will build up in the violet and blue annuli. Some observers also report a circular rotation within these regions; things will begin to “run around in circles”. This image “The Enigma” is by Isia Leviant (1981), Palais de la Découverte, Paris."
Rotating Snake Illusion
Source: http://www.michaelbach.de/ot/mot_rotsnake/index.html and http://www.ritsumei.ac.jp/~akitaoka/rotsnake.gif
"Rotation of adequate figures creates a three-dimensional illusion. It can take some time until the percept emerges. In the example on the right, envisaging a rotating crater may help. The kinetic depth effect (KDE) is a similar related phenomenon." Source: http://www.michaelbach.de/ot/mot_ske/index.html#
"In the neighbouring checkerboard you can see a central “bulge”. This bulge is illusionary: the checkerboard is fully regular and each check is square, as becomes obvious when the small ticks are not present."
"On the right you can see an arrangement of lozenges or elongated diamonds. Question: Where are the diamonds darker, at the top