CEMC Open Courseware (gr. 9-11)
Khan Academy
Grade 7 Material
Desmos
Geogebra
Numberphile
3Blue1Brown
standupmaths
Mr. Bennett
How to use LaTeX
LaTeX Wikibook
Get LaTeX
SageMath
Haskell
Python
Arxiv (pronounced "archive")
New York Journal of Mathematics
Sci Hub