Links
My links
Cool learning resources
General
OpenStax
Khan Academy
Wikibooks Physics
More specific
Note that I have not read all of that myself yet; this list doubles as a reminder for me.
A friendly introduction to group theory
Beginning Mathematical Logic: A Study Guide
Lagrangian Mechanics For Dummies: An Intuitive Introduction
Introduction to Modern Geometry - Axiomatic Method Class Notes
The Mathematics of Quantum Mechanics
Artur Ekert’s YouTube channel
Programming
Misc
Orion’s Arm
excellent infographic on signals in OpenTTD
Anders Sandberg’s pages - check out this guy, really
Atomic Rockets