complete design, synthesis and analysis, of the full scope of Analog and Digital electronic filters: single stage, cascaded stages or in cascaded mixed arrays including A/D and/or D/A converters as needed.
http://www.tatumlabs.com/superFILTER.html
design digital filters of many types, including high-Q notch filters, fractional-sample time delay filters and comb filters, in addition to the more traditional low-pass, high-pass, band-pass and band-stop filters, differentiators and Hilbert transformers
http://www.nauticom.net/www/jdtaft/
calculates the resistor and capacitor values for various op-amp based active filter configurations. Calculations will simultaneously include 7 response curves for each filter.
http://www.wssh.net/~wattsup/audio/afilter.zip
This free online utility, written in JavaScript, helps you to quickly design an active Sallen-Key or Multiple Feedback topology low-pass filter.
http://beis.de/Elektronik/Filter/Filter.html
http://www.ee.washington.edu/circuit_archive/software/filter.zip
Computer aided filter design.
http://www.wssh.net/~wattsup/audio/filtcad.zip
Download Filter Wiz, eSketch, OpFil, WinLAP, a Windows application.
http://www.schematica.com/Fil_Xfer/Transfer.htm
Distributed microwave filters.
http://eesof.tm.agilent.com/