Music Theory 
Music Theory is brought to us by Ricci Adams. This is a lovely educational site. If there was anything you wanted to know about music theory, I’d recommend this be the place you choose to learn it from.
Choose a Lesson, a Trainer, or a Utility from the drop down boxes. I chose a Lesson first. I learned about the Staff, the Clefs, and Ledger Lines. It then took me through a very thorough laymen’s explanation of the subject indicated. I could then continue my lessons or move onto a Trainer.
I chose the Note trainer. I was asked to name the note seen on the staff. And let me tell you I’ve forgotten a lot of my high school music training. It told me how many I got correct, how many I got incorrect and my percentage.
Now for the Utilities, this section is very helpful. You can print Staff paper, you can generate chords and play around with the Matrix Generator.
This is a very nifty musical site that can teach music theory even to me.