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!
Mandolins Heal The World: Don Julin is a master mandolin player, author of "Mandolin For Dummies" and "Mandolin Exercises for Dummies", and one of the best music teacher's I've encountered. I've had numerous private on-line lessons with him, and it's made an impact on all of the instruments I play! Mandolins Heal The World is a treasure trove of lessons with top-notch teachers. Highly recommended!
Banjo Ben: Ben Clark has built a music teaching empire focused on Bluegrass music. There's tons of Tablature of bluegrass songs and fiddle tunes, for Mandolin, Guitar, and the Five-String Banjo. Subscription based, but there's tons of free resources! I'm a Gold Pick Member. :-)
Mandolin Cafe: Scott Tichenor has built a world famous one-stop shop for all things mandolin. There's lessons, mandolin news, interviews, and a fantastic Classifieds section where you can buy instruments, gear, lesson materials and more. I'm lucky enough to live in the same home town as Scott. :-)
Recommended Mandolin Resources