Arts Alive: The United States National Center for the Arts has activities for learning about composers, orchestral instruments, and more! http://www.artsalive.ca/en/mus/index.asp
Classics for Kids: Music games and puzzles, especially great for students in grades Kindergarten through 4th grade. http://www.classicsforkids.com/games/
Creating Music: Music games and puzzles especially fun for students in 3rd grade and younger. http://www.creatingmusic.com/
DJ Spin Doctor: Be a DJ! Combine music samples to create a work of pop music all your own. This is especially fun for students in 4th grade and up. http://www.bbc.co.uk/radio1/fbs/mac/fbsmac.html