Free music notation program
Great artists on each instrument, with recordings of their performances
A great website for learning about your instrument for the first time. Just select your instrument from the menu in the top-right corner, and explore!
"How Playing An Instrument Benefits Your Brain"
One of the best explanations I have ever seen of what learning to play a musical instrument does to your brain, and why it is such an important part of your education. To dig deeper into this, you can also watch this talk by Dr. Sarah Jayne Blakemore that explains how the brain develops during our teenage years. Of course, you only get these benefits if you practice your instrument every day. So go practice! Now! Go! Why are you still reading? Go practice!
Two really good websites for percussion stuff.
Great site for trap-set drumming lessons/videos
The correct embouchure for playing the clarinet, in great detail.
"10 Simple Steps To Improve Your Clarinet And Saxophone Section"
Good stuff here. Everyone should read this and do it.
Jazz At Lincoln Center (New York) "Jazz Academy" page. Good stuff.
Flute Tips blog.
Flute embouchure help for beginners.