supersearches:
sidesearches: physics, philosophy
subsearches: arithmetic, elementary (high school/college) algebra, linear algebra, real analysis, abstract algebra, calculus
Learning (covers many areas of math)
Khan Academy
3 Blue 1 Brown
Brilliant.org articles
Learning (covers one or a few areas of math)
Seeing Theory - "A visual introduction to probability and statistics"
Wrath of Math: linear algebra playlist
Proofs
Proof Wiki
Math Proofs Jrensen
Solutions/Questions
Eqworld - exact solutions to many different types of differential, integral, functional and etc. equations
Math Stack Exchange - Math Q&A website
Human knowledge
Math map (quanta magazine)
Calculators
Desmos
Wolfram Alpha
Math3d
Floating point math
0.30000000000000004.com
LaTeX
Detexify - find LaTeX symbols easier
All LaTeX symbols pdf
Prime numbers
Prime wiki
Prime factorization visualization
GIMPS - Lend your computer to help find (mersenne) primes
Pending
Euler Map Theorem (V-E+F=2) Visual Proof
Wrath of Math: is everybody teaching this math wrong?
Wrath of Math: linear algebra playlist
MIT Open Courseware
Trachtenberg system of multiplication
Math genealogy