Electronics
Includes various reference points to get you started on tinkering hardware.
/r/AskElectronics
A beginner-friendly subreddit on getting help on your electronics projects or anything hardware-related. Just be sure to help them helping you by providing as much necessary details as possible.
/r/electronics
A subreddit on anything electronics-related from news, articles, discussions, or project showoffs.
All About Circuits
A website that is all about electronics and hardware. They offer a wide range of electronic-related materials such as datasheets, calculators, video lessons, articles, a discussion forum, and a textbook.
Awesome Electronics (by kitspace)
An awesome list of various links and references to get started in electronics.
Circuitlab
An online circuit schematics editor and simulator. Supports analog and digital circuits and no installation or registration required to use it.
FreeCAD
A free and open source computer-aided design (CAD) program and modeller created mainly in C++. It can also run on multiple platforms, mainly on Windows, Mac OS, and Linux. If you want to look for the source code, you can take a visit at their GitHub repo.
GreatScott!
A YouTube channel of electronic projects how-tos and tutorials.
Hackaday
A community of hardware hackers mainly helps each other by sharing their projects and discussing hardware-related stuff among them.
Hackster
A community of hardware hackers similar to Hackaday.
Instructables
A do-it-yourself (DIY) community but it also has one of the largest hardware/electronics hackers community out there.
KiCAD
A cross-platform free and open source electronics design automation suite.
KTechLabs
A free and open source IDE for microcontrollers and electronics.
Tinkercad
A free collection of tools by Autodesk to get started on electronics, engineering, and programming. It may look like it's targeted for kids but it's still a good way on getting started with electronics, nonetheless by offering a circuit simulator and an integrated environment for programming and debugging your circuits.
Personal comment:
I recommend it a lot if you're looking for an electronics/hardware reference. Also, they provide a really good starting point on electronics by providing you with various video lessons, articles, and even a textbook with multiple volumes.