algebra

MathBox: Tool for Thought (Algebra & Fourier Analysis)

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

Mathigon
Parallel
Seeing Theory
Euclidea: Interactive geometric puzzles

music

Every Noise At Once

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.

quantum computing

Quantum Computing for the Very Curious

spaced repetition

How to remember anything forever-ish