4 links
ALK Engineering
alkeng.com
Filter Design: Passive radio frequency filters, LC, comb-line, and interdigital. Offers two industry-standard filter design programs, and a freeware program for coil design.
Electrical Engineering Software
geocities.com
Filter design software, microwave, RF, active OP-Amp filters.
Filter Solutions
filter-solutions.com
Software solutions for passive, active, Tx line, IIR, and FIR filter design, synthesis, and analysis.