digital filter simulation tool
http://www.systolix.co.uk/fsintro.htm
design tuned LC circuits and coils
http://members.aol.com/maxfro/
programming tool for Analog Devices AD7725 programmable Sigma-Delta converter
http://www.systolix.co.uk/nf/fwintro.htm
Works nice.
http://margo.student.utwente.nl/el/audio/filtry11.zip
http://www.schematica.com/filter_wiz_files/OpFil.htm
Pulse shaping filter program that finds the pole-zero locations of a transfer function, H(s), for a matched filter.
http://www.digitalCalculus.com/demos/m-freq/
for Matlab and Mathematica. Simultaneously optimizing analog filter designs for any combination of magnitude, phase, and step responses, and quality factors, based on pole-zero locations
http://www.ece.utexas.edu/~bevans/projects/syn_filter_software.html
A real-time variable audio filter Windows utility.
http://www.wssh.net/~wattsup/audio/filter.zip