Music Creation and Search Sites
Myna- http://www.aviary.com/tools/myna
Use Myna to remix music tracks and audio clips.  This is very similar to Garageband and a nice alternative for those who do not have the Apple application.

Soundation - http://www.soundation.co Soundation Studio is a powerful tool for creating music online.  Very easy to use, with a drag/drop interface for creating excellent-sounding music. Can be a little scary for new users so check out learn Soundation http://www.soundation.com/learn

Jam Studio- http://www.jamstudio.com/Studio/index.htm   Fun site!  Has help videos on YouTube.  This site lets users select a "key" to play in and a theme to match.

Loops Labs - http://www.looplabs.com/ Nice time- line editor; learning curve is a bit higher than most sites, but is great for music mixing

Creating Music http://www.creatingmusic.com/  - Very user friendly website shows kids how to create digital music,  It is a place for kids to compose music, play musical performance and music games and puzzles..

iNudge-  http://www.inudge.net/index.en.html Fun site for kids, who can make music by drawing shapes.

Glitchscape-http://www.glitchscape.com/ Create sound by drawing vector shaped rectangles.
Find Sounds www.findsounds.com
This site serves as a search engine to more than 1  million sounds effects and instrument sounds found on the web. 
Freeplay Music www.freeplaymusic.com
This is the place to go if you need free, copyright-cleared, quality music for any presentation.
Exploring with Music:
Play notes and chords online  (K-12)
Play notes and chords online  (K-12)
Online Video Piano Lessons http://www.pianonanny.com/1.html
Global Groovin http://pbskids.org/mayaandmiguel/english/games/globalgroovin/game.html   
Mixing music using sounds from instruments from all over the world (K-6)
Hand Symphonyhttp://handsonlycpr.org/symphony/?id=214b  
This activity, sponsored by the American Heart Association, allows students to experiment with looped, rhythmic hand clapping, snapping and rubbing pattern combinations for Dance, Hip-hop, and Latin musical styles. Applications can include student arranging and rhythmic dictation (K-12)
Create your own interactive beats and rhythms  (K-12)
Four part interactive activity; Learn names of percussion instruments, match sound to an instrument, listen and copy a rhythm, recognize the instrument that is not playing  (K-3)
Learn about and listen to instruments, play virtual instruments, and sing-a-long songs  (K-6)
Learn about the instrument families and hear their sounds  (K-12)