Yiping Lu's Linear Algebra Cheat Sheet is a great review sheet for matrix operations and transformations.

MiniReference's Linear Algebra in 4 Pages summarizes the key formulas on a few pages.

Jiří Lebl's Notes on Diffy Qs is a solid free textbook for differential equations.

UNCW's ODE Reference Sheet is handy for quick formula lookups.

The Falstad Circuit Simulator lets you build and visualize circuits directly in your browser.

All About Circuits has clear explanations for circuit theory and analysis.

MIT's Circuits and Electronics (6.002) has good problem sets and lab materials.

Sanfoundry’s 50,000+ Electrical Engineering MCQs is a massive bank of practice questions.

Behzad Razavi’s YouTube channel is amazing for learning microelectronics and circuit design.

Analog Devices’ Electronics Course wiki has great practical lab materials and circuit design notes.

The Feynman Lectures on Physics (Vol. II) is great for building real intuition for electromagnetism and Maxwell's equations.

David Tong's Electromagnetism lecture notes from Cambridge are fantastic.