This excellent demo comes from David Anson, a student at Cornell. His other demos for the project in which he produced this are at his web site.
Mach Banding Interactive Demonstration - A phenomenon whereby piecewise linear intensity changes across a polygon boundary trigger the human visual system into perceiving the boundary as a bright band (3D Computer Graphics by Alan Watt, page 132). 

The Java applet below lets you experiment with this phenomenon. Play around with the settings to see what effect they have on your perception of the Mach Banding. Different combinations of colors, bar widths, and the presence of separators have a big impact on the eye's perception of the image. 

NOTE: The applet is reasonably intelligent about updating the display (try hitting "Enter" after making a change), but you can force a repaint at any time by pressing the "Repaint" button. 

This applet requires Java 

A project description is available.