Google Music Lab
This is a great site with lots of fun, simple music activities. Kids are able to create their own music, rhythms, and more.
Incredibox is a web app that allows you to create beatboxing rhythms. Just drag a picture from the bottom onto one of the characters.
Isle of Tune
Create your own musical town. Create a road, then add houses, trees, and other features. After you place an item, click on it to choose the sound it will play. Once you put a car on the road it will play the sounds you selected as it passes each item.
This is a great game to learn where the notes are on the music staff. Just like whack-a-mole, but with music notes.