Online program for exploring the effect that different windows (and number of taps) have on the frequency response. A fun way to interactively explore design techniques. Lowpass filter only.
http://web.mit.edu/6.555/www/fir.html
http://www.eagleware.com/
demonstration program plots harmonics in time domain. in german.
http://www.cetest.nl/zips/fourrsy.zip
Free online program for designing several types of active filters, including low-pass, bandpass, and high-pass, with Bessel, Butterworth, Chebychev, inverse Chebychev, or elliptical passband responses. You can even choose the specific op-amp to use, or a
http://www.chipcenter.com/oltu/afw/af.html
a spectrum analyzer program for DOS, Linux, Linux/X, and Sun Copyright (C) 1995 Philip VanBaren.
http://www.wssh.net/~wattsup/audio/freq5.zip
Microwave and RF filter design software. Lists analysis and synthesis capabilities with examples of software output.
http://www.waveconsoft.com/
http://www.mwoffice.com/products/txline.html
Microwave and RF filter and multiplexer design software. Provides detailed information about the product.
http://www.eee.metu.edu.tr/~nyil/filpro.html