Digital Filter Plus is an Finite Impulse Response (FIR) and Infinite Impulse Response (IIR) digital filter design program that runs under Windows and Linux.

Finite Impulse Response Filters
Windowing
Parks-McClellan Remez Exchange Algorithm
Hilbert Transform
Raised Cosine
Square Root Raised Cosine
Gaussian
Infinite Impulse Response Filters
Pole Zero Placement
Butterworth
Chebyschev
Inverse Chebyschev
Elliptic
Bessel
Bilinear transform
Matched z-transform