Fourier is an applet which demonstrates Fourier Series and frequency domain concepts. It was written using Java JDK1.1. If you do not get a running applet, there is a good chance your browser blocks the execution of Java applets or has not Java Virtual Machine at all.
Unblock it or follow and download the most recent Java Plug-in 1.5.0_01 (yellow button in the upper right corner).

Please be patient while the applet loads.