If you read my Prezi a while back, you know that I was a music major a bazillion years ago! When I read about this site at Free Technology for Teachers, I was intrigued!
MusicTheory.net has all sorts of interactive music lessons and exercises for all levels of musicians, from beginners to advanced. Music teachers will find all sorts of great tools too!
Be sure to take a look in the Tools section. You will find several different "musical calculators" and an easy way to print your own musical staff paper (with or without the treble and bass cleff symbols). My favorite tool is the Pop-Up Piano where you can play right there on the screen!
If you are a music teacher (or even a former music major!), be sure to check MusicTheory.net out and let me know what you think!