Mathematics

Lemma by Dr. Pavel Grinfeld

Linear algebra course

Singular value decomposition

Excellent resource on FFT

Physics

Physics Forums to ask and read other people's queries

PhysLink

Abbe School of Photonics

Photonics Wiki

RP Photonics

Engineering

Tutorial on electronics

Scholarpedia

How to take blood pressure

Measure blood pressure (Korotkoff sound)

MRI

Q&A on MRI

The Efficient Engineer YouTube channel (Poisson Ratio)

Magazine

phys.org

Nature

Science Magazine