FFT Page - Links and various Fortran FFT implementations.
fftpack - A package of Fortran subprograms for the fast Fourier transform of periodic and other symmetric sequences.
FFTW - Fast free C FFT library; includes multi-dimensional, real-complex, and parallel transforms.
Jens J. Nielsen's Home Page - Arbitrary length FFT C-source, 3D gravity simulator, quantization of FIR filter coefficients. C source.
Joerg's FXT Page - Source code and links for many different FFT implementations and related information.
Row-oriented Multidimensional Fast Transform Library (RMFTL) - Power-of-two real and complex FFT software that is faster, and more accurate, than conventional iterative implementation of one-dimensional FFTs. Based on a proprietary algorithm.
Sigview - FFT Analysis Software - A real-time spectral analysis application with spectral analysis tools, statistical functions and visualization system. It can analyse data from WAV files or sound input from microphone or CD player. Shareware.