Links to many sources of FFT code and information.
http://www.fftw.org/links.html
A simple C++ R-tree implementation with Segment Incidences Detector as an application.
http://www.volny.cz/r-tree/
Lot of categories and links.
http://www.geom.uiuc.edu/software/cglist/
Contains Fast Fourier Transform source code (C, C++, Pascal), tutorial, and math theory.
http://www.intersrv.com/~dcross/fft.html
A collaborative effort to develop a robust, easy to use, and efficient C++ software library of geometric data structures and algorithms.
http://www.cgal.org/
http://www.fftw.org/
A program for exploring topological knots and knot-like objects in a 3-D rendered environment offering stills, animated knots (including dynamic forces) and demos. Aimed at mathematician and artist alike.
http://www.cs.ubc.ca/nest/imager/contributions/scharein/KnotPlot.html
By Paul Bourke. Covers both the analog Fourier transform and the discrete Fourier transform (DFT), including the math and various properties. Introduces the FFT.
http://astronomy.swin.edu.au/pbourke/analysis/dft/