The Mother of All Free Filter Software Websites: Iowa Hills Software - RF Cafe
SVC Filter Design Program
Nooelec - Distill:FM Barebones - Broadcast FM Bandstop Filter for Software Defined Radio Applications - Other RF Blocks - SDR Addons - Software Defined Radio
Free RF simulation software - AD5GG
A Primer on RF Filters for Software-defined Radio - OneSDR - A Blog about Radio & Wireless Technology
MATLAB Filter Design Wizard for AD9361 [Analog Devices Wiki]