VisionLab (VLAB on the Windows Start menu) has both demonstrations and experiments.  In both the demonstrations and experiments you can change most parameters to see the effects on perception.
 

Demonstrations include:

 
Apparent Motion
Stereo Forms
Random-dot Stereograms
After-effects
Subjective Contours
Equiluminance
Color Assimilation
Illusions
    Ponzo
    Poggendorf
    Muller-Lyer
Kinetic Depth
Tachistoscope

Experiments include:
 

Line Length
Muller-Lyer
Spatial Frequency Shift
Dot Detect Discrimination