Theory
Rhythmic light by Fred Collopy
https://rhythmiclight.com/
The most full and honorable source of knowledge about light to sound correspondences. Started in 1998 and still growing!
Encyclopedia of Microtonal Music Theory
http://www.tonalsoft.com/enc/encyclopedia-index.aspx
A very deep resource about all the subtleties and complexities of music theory by [Joe Monzo](http://www.tonalsoft.com/enc/m/monzo-writings.aspx)
Music animation machine
http://www.musanim.com/
A life long project of Stephen Malinowski – he manually animates music pieces into nice visual playalongs. He uses blue color for the tonic (C) and accending colors for every next step of a fifth.
Home page of William A. Sethares
https://sethares.engr.wisc.edu/index.html
A very important figure in sensory dissonance research. His book "Tuning, Timbre, Spectrum, Scale" gives a comprehensive understanding of many important music concepts in a scientifically based way.
Acoustics and Vibration Animations by Dr. Daniel A. Russell
https://www.acs.psu.edu/drussell/demos.html
Teaching Professor of Acoustics, Graduate Program in Acoustics, The Pennsylvania State University. He created animations illustrating acoustics and vibration, waves and oscillation concepts.
Music acoustics
http://www.phys.unsw.edu.au/music/
Physics and music have been related for millenia. The art and science of music acoustics are presented here, in musician-friendly format, as is our research in music science.
CIELAB.XYZ
https://cielab.xyz/colorconv/
Online color converter for different color models
List of chords
https://en.wikipedia.org/wiki/List_of_chords
Comprehesive Wikipedia collection of chords
List of chord progressions
https://en.wikipedia.org/wiki/List_of_chord_progressions
Comprehesive Wikipedia collection of chord progressions
List of scales and modes
https://en.wikipedia.org/wiki/List_of_musical_scales_and_modes
Comprehesive Wikipedia collection of pitch class collections
The Music notation project
http://musicnotation.org/systems/
Many people struggle to learn to read and play music, and many give up before they become proficient. Could a better notation system make reading, writing, and playing music more enjoyable and easier to learn? We think so.
The Exciting Universe Of Music Theory
https://ianring.com/musictheory/scales/
A complete study of all Scales in the existence by Ian Ring. "The place for all you music theory nerds to geek out. Bask in the warm bath of wisdom, and be envied by all your peers with your deep knowledge of musical lore. "
AllTheScales.org
http://allthescales.org/
Based on the Western system of notes (as exemplified by the piano keyboard), there are 1490 possible scales. 2 In other words, with the combined genius of medieval monks through Beethoven through Stravinsky and Coltrane, we've managed to explore roughly 1% of this musical terrain.