Musicards ... "an ever-growing collection of highly-customizable online music theory flash cards. These music flashcards can be used by beginning music students who are becoming acquainted with key music rudiments, all the way up to intermediate and advanced students."
Another very handy diagram for music teachers!
How did Beethoven continue to compose even as he was going deaf? A look into the math behind his music
Piano Chord Chart
Kindergarten Worksheets - all the ones I have looked at and printed so far are free. Not sure if you might have to pay for some of them. I have used the different math concepts worksheets on here thus far and they have worked great with my 5 year old.
Painting with Watercolors, Glue and Salt
An online composition tool, similar idea to GarageBand in selecting and layering samples. Can choose guitar chord progressions, different instruments. Not hugely intuitive interface but a few minutes fiddling around will pay off.