This java applet is a simulation that demonstrates standing waves on a vibrating string (a loaded string, to be precise).
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.