This java applet is a simulation that demonstrates scalar waves (such as sound waves) in two dimensions. It demonstrates the wave principles behind slit diffraction, zone plates, and holograms.
To get started with the applet, just go through the items in the Setup menu in the upper right.
Also see the Ripple Tank applet.
Full Directions.
The source.
More applets.
Zip archive of this applet.
http://www.falstad.com/wave2d/