Think blocks A and B are different colors?
Daggone. (click to see animation)
An oldie but a goodie - flashing spots (or a "scintillating grid") where there aren't any! Avert your eyes if you get migraines, because this might set one off.
Find the green dot - just stare at the crosshatch:
And lastly, one that doesn't rely on your visual cortext to freak you out (though some would argue that for it to BE an optical illusion it must do so), but rather does so just by being freaky:
One person? 2 people? or 3? See the dog? The tree? Hmmm? Do ya? DO YA?