Make your own wave forms interfere constructively and destructively http://mysite.verizon.net/vzeoacw1/wave_interference.html
Ripple Tank for Waves http://webphysics.davidson.edu/Applets/Ripple/Ripple_JS.html
Light Interference http://vsg.quasihome.com/interfer.htm
Beat Frequencies http://library.thinkquest.org/19537/java/Beats.html
Transverse and Longitudinal Waves http://www.cbu.edu/~jvarrian/applets/waves1/lontra_g.htm
A Doppler effect applet http://www.explorescience.com/activities/Activity_page.cfm?ActivityID=45
Try the "Waves on a String" Applet http://phet.colorado.edu/web-pages/simulations-base.html
Gas discharge spectra http://www.enstimac.fr/~latailla/java/spectre/
Another Doppler effect applet with sound http://library.thinkquest.org/19537/java/Doppler.html
The Physics of Rainbows http://www.phy.ntnu.edu.tw/java/Rainbow/rainbow.html
Refraction of Light at a Boundary http://www.sciencejoywagon.com/physicszone/otherpub/wfendt/huygens.htm
What's happening to the fish-eye view? http://www.phy.ntnu.edu.tw/java/optics/path_e.html