I am looking at this thread as being an ongoing list, and summary, of various DSP tools, Equalizers and other plug-ins for audio/music playback (or mastering/recording) - be they commercial/paid or free/open-source.
Note: Plug-ins require music-player software that can host them. There are some standard formats such things in Windows and on macOS. For macOS the most common is the “Audio Unit” (AU), and Windows is generally “VST3”). Players like JRiver Media Center and Audirvana can both host these plug-ins, and both are available on Windows and macOS.
Commentary, discussion and questions are welcome.
I expect that many posts here on specific plug-ins will spawn dedicated, linked, threads to cover them in more detail. But this is a starting point for cataloging and high-level commentary.