Wokwi

Web based microcontroller simulator. Great for embedded design with limited resources!

Analog Design Tools

Analog Design has a bunch of free calculators and simulation tools.

Mark Fowler

A professor that I found online with free to access Powerpoints on Signal Processing.

Studying for Principles and Practice of Engineering (PE) Exam for Electronics, Controls, & Communication

This is a Discord server with lots of people who are studying for the ECC PE. In the channels are people who can help explain problems as well as other online resources