EngineersFilter by rs-met is a free filter plug-in that allows for extremely steep filtering. It achieves this by implementing high order IIR filter design methods that are commonly used in science and engineering, namely Butterworth, Chebychev, inverse Chebychev and elliptic (aka Cauer) filters. Each of these filter design methods can be applied to create low-pass, high-pass, band-pass, band-reject, high-/low-shelving and peak/dip filters of orders up to 20.
EngineersFilter is available for free as a VST effect plug-in for Windows.