Links:

Fourier Series

Build your own Fourier Series (Excel file)

More Links:

• JAVA APPLETS
  Adjustable Fourier Series Applet
  Fourier Series Applet with Sound
  Fourier Series and Music
  Zoom in on a Square Wave approximation
  Sampling and Fourier Series 
• ONLINE TUTORIALS
  From SOS Math
  From Wolfram Research
  From University of Ulm