Fast Fourier Transforms
February 2000 - Don Cross
A short tutorial with free source code for how to compute the fast fourier transform (FFT).
The DFT "à Pied" - Mastering the Fourier Transform in one Day
November 1999 - Stephan M Sprenger
This article gives a very easy to understand explanation of the DFT (Discrete Fourier Transform) and also gives example source code for computing both DFT and FFT (Discrete Fast Fourier Transform).
Fast Fourier Transform (FFT)
This is in fact chapter 12.2 in Numerical Recipes. This chapter describes the FFT algorithm and gives an example implementation of it. It also mentions briefly how the FFT algorithm can be implemented to run more efficiently.
The FFT Demystified
This article describes in detail several FFT algorithms, decimation in time, decimation in frequency, radix 2, radix 4, etc.