I have always been intrigued by optical illusions.   Each time I find something new I spend a lot of time trying to understand what makes it work.   Arthur Shapiro has come up with a new illusion that also shows how our eyes and brain perceive light.  What I really enjoyed about this write-up is the ability to tweak the illusion to see how changes affect what you see.  Check it out here:

http://scienceblogs.com/cognitivedaily/2008/02/what_illusions_tell_us_about_t.php