FFT :-
Tutorial
FFT @wikipedia Link
The Scientist and Engineer's Guide to Digital Signal Processing Link
Mathematics of the Discrete Fourier Transforms Link
Picture Book of Fourier Transforms Link
The FFT Demystified Link
Source Code
FFTReal :: C++ and Delphi class for computing FFT and IFFT on vector and real numbers Link
NFFT :: C subroutines library for computing non-equispaced DFT Link
dgbfft :: Extremely fast for floating point FFT Link
FFT codes and related stuffs Link
FFTPACK from netlib Link
vfftpack from netlib Link
FFTW Homepage Link
benchFFT :: To benchmark FFT softwares Link
C-source for a very fast arbitary N FFT routine Link
ccSHT :: A fast parallel spherical haramonic transform Link