Let your eyes sweep around the above, and try to pinpoint the black dot.
The lines sometimes look a little crooked.
(This optical illusion is an HTML version of this picture, there are no pictures on this page.)

• The Exploratorium has a similar shimmer effect; which is not at all like spirals.
• Akiyoshi's Optical Illusions
• McCollough effect
• OptiIllusions.com
• Psych Illusions