Any suggestions are welcome. Post suggestions here. I will gather the details and add them. Thanks.

This will be organized as results come in.
@sweetchaos made a guide for
PEQ filter data input and operation.
For crossover software
go here.
Linux:
[GUIDE] How to use Equalizer APO (PEQ) presets in Linux.
Hang Loose Convolver (HLC) is a specially designed stereo and multichannel convolver to compare “level matched” convolution filters in real time with “instant switching” between filters.
PulseEffects signal processing tools including parametric equalizer.
PulseEffects has been renamed
EasyEffects as recent versions have moved from using pulseaudio to requiring pipewire.
JamesDSP Audio Effect Processor For PipeWire Or PulseAudio (Equalizer, Bass Boost, More).
CamillaDSP supported platforms: Linux, macOS, Windows.
HiFiScan helps equalize an audio system to get the best possible audio quality from it. It can be automatic or manual.
Camillagui runs in the browser and handles the actual interface.
dsp is an audio processing program with an interactive mode.
Daphile is an audiophile class music server & player OS – targeted to dedicated headless PC.
moOde audio player parametric and graphic equalizers with CamillaDSP and music playback for Raspberry Pi family of single board computers.
Volumio audiophyle music player for Rasberyy Pie.
FusionDsp a Volumio 3 plug in
JRiver comprehensive and seamless software available for managing and playing your media.
Voicemeeter audio mixer application w/ virtual audio device used as virtual I/O to mix/manage any source from/to any audio devices or applications.
DSP for Volumio (previously Brutefir3)
BruteFIR - the convolution engine used by daphile, Volumio's DSP plugin, an LMS plugin and probably others, also manually configurable in a custom alsa config, JACK etc. You can also use it to make crossovers.
BrutefirDRC - the LMS plugin that uses BruteFIR running on the server, configurable per endpoint. Archimago has a
howto.
Jconvolver - formerly Zita-convolver, the convolution engine used in PulseEffects, which can also be used with JACK.
Alsaequal - an equalizer you can add to the alsa config. Basic usage is a fixed band graphic equalizer, but it can be used to give a control interface to any other LADSPA plugin via the usual alsa mixer interface. The normal alsa config for ladspa plugins only allows fixed configurations, although for something like an active crossover that's probably an advantage.
REW is free room acoustics analysis software for measuring and analysing room and loudspeaker responses.
oratory1990 is headphone specific equalization.
AutoEq uses EQ profiles to be used with EQ software.
foobar2000 is an advanced freeware audio player with EQ runs under wine.
Shairport Sync uses DSP to allow the user to manually input the REW PEQ settings and save for later.
HQPlayer PEQ, a convolver with impulse response files, cross-feed and multi-channel support/mixing/routing.
Pulseaudio Parametric Equalizer is a python GUI to insert a 3 band PEQ w/ high/low shelves into the pulseaudio audio server.
Pipewire do
post-processing of the sound generated by a application, a possible drop-in replacement for both Jack and Pulseaudio.
Calf plugin suite.
LSP (Linux Studio Plugins) is a collection of open-source plugins.
DeaDBeeF is a modular audio player with a 18-band graphical equalizer and other DSP plugins.
Plus there's a whole host of LADSPA plugins you can use via JACK, alsaequal, or with a fixed configuration in the alsa config file. A good example is the
PE32LR parametric equalizer from the
Linux Studio Plugins Project which provides the parametric equalizer for PulseEffects.
macOS: (System wide parametric EQ solutions on OSX typically rely on separate plugin hosting software and the actual plugin which does the actual equalization.)
Audio plugin hosts include:
Hang Loose Convolver (HLC) is a specially designed stereo and multichannel convolver to compare “level matched” convolution filters in real time with “instant switching” between filters.
MenuBus has a free version but is no longer actively developed.
Neutron Music Player audiophile-grade platform-independent
32/64-bit audio engine, parametric EQ.
eqMac parametric EQ and mixer.****
SoundSource w/ EQ profiles for headphones from Oratory, Reference Audio Analyzer, Crinacle, Inner Fidelity & headphone.com & 20 minute trial period.****
HiFiScan helps equalize an audio system to get the best possible audio quality from it. It can be automatic or manual.
Hosting AU with
BlackHole (BlackHole: Virtual Audio Driver. Setting up the EQ details for macOS
are here.) or
Soundflower can be used as a system wide AU plugin host.
JRiver comprehensive and seamless software available for managing and playing your media.
EQ plugins include:
MFreeformEqualizer draw the equalization that you want and the plugin will make it happen.
Waves Q10 plugin as a PEQ solution.
Voxengo PrimeEQ is a parametric EQ plugin but is not free.
Fabfilter Pro Q3 is another parametric EQ plugin, regarding setting the Q values please refer to comment
#305 and
#308
LAConvolver plugin is a free convolver EQ which works with impulse response WAV files.
EQE is a system wide parametric equalizer on iOS but requires jailbreaking.
Element is a VST/AU host and comes with quite a few built in effects as well.
SplitEQ parametric EQ with 8-bands and proprietary effects.
MathAudio add-ons Room EQ VST, VST3, AU and Foobar2000 component.
Rogue Amoeba SoundSource headphone profiles from headphones.com and Crinacle + others.
QRange 12 band EQ, each band can be used as peak, shelf and cut filter.
REW is free room acoustics analysis software for measuring and analysing room and loudspeaker responses.
foobar2000 is an advanced freeware audio player with EQ.
VLC is a free and open source cross-platform multimedia player w/10 band EQ.
CamillaDSP supported platforms: Linux, macOS, Windows.
Universal Audio Equalizer for Mac OS X
Boom 3D Desktop 31-band equalizer, 3D surround.
eqMac 10 band EQ.
Sonarworks Reference 4 calibrates your studio monitors and headphones.
Audirvana can do bit-perfect and exclusive access but can't do system level hosting.
Audio Hijack can host system level DSP, including those EQs.
ToneBoosters Equalizer 4 all-purpose dyamic equalizer with AI assist functionality.
Toneboosters TB Equalizer v3
Toneboosters Morphit hundreds of headphone profiles.
FabFilter Pro Q 3 up to 24 EQ bands.
oratory1990 is headphone specific equalization.
CanOpener Studio EQ with Crossfeed plugin.
AU Lab the download link is at the bottom of the page.
Boom3D and Boom2 10-band & 31-Band Advanced Equalizers.
HQPlayer PEQ, a convolver with impulse response files, cross-feed and multi-channel support/mixing/routing.
Soundsource Per-App Audio Effects with the built-in 10-band equalizer.
MEqualizer is an extremely easy-to-use and powerful 6-band equalizer.