Google's Creatability suite...
....lets you play the marimba with your nose and so much more.
This site has a whole bunch of rhythm training activities.
This site lets you practice your rhythmic dictation.
Here's a good site for fingering and note reading practice. :)
The Rhythm Randomizer generates a new rhythm within parameters you specify. Try clapping them- if you are successful, you can make them more complicated or longer in the options. They're also good to choose rhythms for playing scales if you want to mix things up.
Ever wanted to compose, but were worried about getting all the notes and rhythms right? Mario Paint Music Composer takes all the anxiety out- now you can compose with frogs, lightning clouds and skulls. Email your best attempt to mccarrey_alison@asdk12.org so I can see what you did!
This site is really fun to play around with. I like the Song Maker and Melody Maker, particularly.