Musical U: This website is headed by a great music educator named Christopher Sutton. It's an outstanding resource for all things related to Music Theory, including ear training, scales, chord building, the Circle of Fifths, and more! It's subscription based, but there's tons of free resources!
Recommended CGDA Resources
The Bad-Ass Tenor Guitar Register: If you've been looking for tenor guitar/tenor banjo/mandola (CGDA) lessons, you know they're not easy to come by. Jeff West from Humble Baritonics has made a new site where you can find lots of lessons in one spot. Knowing Jeff, he'll be adding more constantly!
Suggestions?: If you know of a great resource that Fretboard Toolbox users might find helpful, please let me know on my "Contact" link, which you can reach by clicking "Suggestions".