Auto Diagnostics and Publishing web site. They publish good YouTube training videos, teach you how to build your own test equipment, and sell unique automotive diagnostic tools.
When placing an order, they will most likely have an expert available to help you purchase the proper parts.
A leading source of factory diagnostic and repair information for over 30,000 vehicles between 1983 to current.
Even if you don't own a fluke, the sections describing tests and how to interprete results are very informative.
Able to view people's complaints for Year/Make/Model. You can also access Recalls and TSBs.
A general list of troubleshooting procedures to help the DIY mechanic get going on an issue
One of the many great welders on YouTube. He taught me that MIG welding is something you feel and listen to.
A leading source of factory diagnostic and repair information used by 200,000+ technicians.
A Professional Mechanic that shares his experience, his daily jobs, and tips and tricks.
A Chemical Engineer that is an advanced DIY automechanic. He really pushes the concept of not being a parts changer.
An off-the-cuff Video Blog about Electronics Engineering, for engineers, hobbyists, enthusiasts, hackers and Makers
Under the [Technical Articles] section, numerous PDF files can be downloaded with great automotive mechanic content.
I utilize Paul Falstad's Circuit Simulator to play around with circuit designs to understand how they work.