I keep talking about these to people and ranting about how this is the best way to learn and intuitively understand certain things, so if you ended up on this page it's probably because I gave you this one link for the sake of efficiency. Have at it! These are all worth digging into.

Collections:

Explorable Explanations
a hub for learning through play

https://ncase.me/

http://tobyschachman.com/

https://bost.ocks.org/mike/

Bret Victor, beast of burden
Bret Victor has been provided by the management for your protection.
Bret Victor

Math:

Red Blob Games
Interactive tutorials for math and algorithms
Blobs in Games

http://setosa.io/

https://betterexplained.com/

http://polytrope.com/

https://jackschaedler.github.io/

http://joshuahhh.com/

Music:

Learning Synths
Learn about synthesizers via Ableton’s interactive website. Play with a synth in your browser and learn to use the various parts of a synth to make your own sounds.
Chromatone: visual approach to music
Colors and notes combined in a scientific way to use in music education, exploration, practice, production, performance and collaboration. Open source.

Tools:

The magic notebook for exploring data / Observable
Discover insights faster and collaborate seamlessly with interactive Observable notebooks built for data analysis, visualization, and exploration.

Theory:

https://andymatuschak.org/

https://hamishtodd1.github.io/