Here are two more resources that I picked up at a session with Tammy Worcester at ISTE. http://www.tammyworcester.com/ These are great for music teachers but I can see some application in other areas as well. The first one is http://www.jamstudio.com/ which allows you to create, edit, and download your own music. Even a novice musician can create some great music. Personally, I think of some of the little movie projects my students do and how cool it would be to let them spend some of their free time in class or at home creating some theme music that is authentic and theirs. I also plan on using this site to have my school News Show crew create our own personalized anthem. I am a PC guy and I hear this is a similar to Mac's garage band.
The other site is more specific to music and really to a piano player. When you go to http://www.kisstunes.com/ you can turn your keyboard into a piano which is pretty sweet!