algebra

MathBox: Tool for Thought (Algebra & Fourier Analysis)

calculus

An Interactive Introduction to Fourier Transforms

complexity science

Complexity Explained

cryptography

the cryptopals crypto challenges

drawing

ZygoteBody for learning human anatomy

geography

The True Size Of Nations

learning

How to remember anything forever-ish

machine learning

Neural Network Playground

mathematics

An Interactive Introduction to Fourier Transforms
Euclidea: Interactive geometric puzzles
Seeing Theory
Mathigon
Parallel

music

Every Noise At Once

music theory

Phrases and Cadences

philosophy

The History of Philosophy

physics

Physics Classroom

programming languages/clojure

4Clojure

programming languages/python

Python Tutor | Visualize Python Code Execution
CodeSkulptor3
CheckiO | Python Online Playground

programming languages/r

Swirl: Learn R, in R.

programming languages/scratch

Scratch Homepage

quantum computing

Quantum Computing for the Very Curious

spaced repetition

How to remember anything forever-ish

transform calculus

An Interactive Introduction to Fourier Transforms