Introduction To Fourier Transform
- A description of the principle of fourier transform and subsequent derivation of formulae. Page includes graphs.
- http://www.rnowik.com/document.php?id=7

Fast Fourier Transforms
- Survey and history of FFT algorithms from Wikipedia.
- http://en.wikipedia.org/wiki/Fast_Fourier_transform

An Introduction to Fourier Theory
- A mathematically inclined introduction with illustrations and explanations of related theorems, its properties and numerical methods for calculating transforms.
- http://gershwin.ens.fr/vdaniel/Doc-Locale/Cours-Mirrored/Maths-Stuff/fourier/

Introduction to the Fourier Transform
- Notes on applications to Image Processing.
- http://www.cs.unm.edu/~brayer/vision/fourier.html

Mathematics of the Discrete Fourier Transform (DFT)
- Notes by Julius O. Smith III, Stanford.
- http://ccrma-www.stanford.edu/~jos/r320/

Fast Fourier Transform
- Article in MathWorld.
- http://mathworld.wolfram.com/FastFourierTransform.html