An Interactive Guide To The Fourier Transform

An Interactive Guide To The Fourier Transform

http://betterexplained.com/articles/an-interactive-guide-to-the-fourier-transform/

iPhoneFFT

https://github.com/alexbw/iPhoneFFT

A simple FFT library for the iPhone, based on Ooura's FFT code. You should now use Apple's built-in Accelerate library. This was made before Accelerate was released, so it's now irrelevant. — Read more 

FFT Links

Several FFT Links

http://www.kurims.kyoto-u.ac.jp/~ooura/fftlinks.html