http://ourworld.compuserve.com/homepages/f_ostrander/pfilterw.htm
digital filter synthesis tool
http://www.systolix.co.uk/feintro.htm
Specializes in filter design software tools and consulting foractive, passive, and digital electronic filters. Description of company and presentation of the software. Download possible.
http://www.omicronsw.com
calculates component values for different orders & mathematical approximations (i.e. Butterworth, Linear-Phase) of passive crossover networks. A schematic is displayed with component values. All inductor values are in milli-henries & all capacitor values
http://www.wssh.net/~wattsup/audio/pxover.zip
Both LC and microstrip filter design. Free demo version available.
http://www.alkeng.com/
FIR and IIR filter design.
http://www.mds.com/
Designs a Digital Filter and draws the frequency response!!!
http://dolphin.wmin.ac.uk/filter_design.html
Active Filter Designer
http://www.schematica.com/