Light Rectangle Illusion
What to see and do: The inside rectangle looks as though one end is darker and the other is lighter, right? Well, it is not. If you drag the inside rectangle to the white space below, you will see that it is all one color.
What's happening: The different colors in the outside box, makes the gray rectangle look darker in one side and lighter in the other.
Credit: This illusion was based on an illusion in "Incredible Visual Illusions" by Al Seckel.