Motion aftereffects or illusions are among the strongest aftereffects that can be perceived and many such illusions exist. Al Seckel offers up some good illustrations of illusions along with excellent explanations. Mark Newbold provides a clever Java applet which forces the viewer to reverse the Necker cube repeatedly. While you are at this site, try a few of his other illusions as well. The famous San Francisco Exploratorium offers an interesting selection of online exhibits relevant to perception.
Audition is another well-researched sense.
The Acoustical Society of America was founded in 1929 to increase and diffuse the knowledge of acoustics and promote its practical applications. This site introduces the ASA and gives a good introduction to the physical characteristics of sound. Dr. Alec Salt of the Department of Otolaryngology, Washington University School of Medicine in St. Louis, has put together an excellent introduction to the functioning of the inner ear. This is a spectacular list of links maintained at the Technische Hochschule Darmstadt. It is concerned with all aspects of links on audition, ranging from links to databases and tutorials, tinnitus, bioacoustics, speech, and audio hardware and physical acoustics.