If plugins were the fun side of DSP, measurement was the honest side. You can convince yourself a filter sounds good; you cannot argue with its magnitude response. Over the years I built a small arsenal of measurement tools and published a collection of measurements made with them.
The VST Plugin Analyser
The tool this site is probably still most visited for: the VST Plugin Analyser dissects a VST plugin from bottom to top — impulse and frequency response, phase and group delay, distortion type identification, static characteristics, attack/release envelopes, peak/RMS tests, and a free measurement workbench for building your own procedures. Plots can be stacked and exported to PNG or SVG. It is still linked from audio forums today, which is why its URL will keep working forever.
Deconvolver and friends
The Deconvolver answers a simple question: given a dry and a wet signal, what transfer function connects them? Strictly it only works for linear, time-invariant systems, but with a window applied it reveals a surprising amount about non-linear devices too — I once identified a mystery device’s internal upsampling purely from its cut-off behavior near Nyquist. There was also a DUnit-based VST-Plug-In Unit Test that ran automated “extreme programming” style checks over whole plugin collections — quality assurance for my open source project.
Measurements worth keeping
The measurements page collects results: an interaural crosstalk transfer function averaged over an HRTF database of about 50 people (needed for the Headphone Mix plugin), an analogue C-weighting filter compared against its bilinear-transform implementation, lowpass characteristics with group delay plots, and a full static/dynamic characterization of the Behringer Composer — gate, key filter, limiter, compressor and release curves, measured out of pure curiosity.