To set the string in motion, click "Center Pluck" or "Fundamental", or click on the string.
Below the string you will see a graph showing each normal mode's contribution to the string's vibration. There are two sets of bars; on top are the magnitude bars, which shows the amplitude of each normal mode. These bars can be adjusted with the mouse, or you could double-click on one to isolate a particular mode.
This applet has sound if you are using java 2. If you don't see a "Sound" checkbox, then you should get the Java plug-in.
Rectangular membrane applet.
Bar waves applet.