Do you like to dance and sing - of course you do!
I've just found two open source projects that let you do just that.
Stepmania - If you have ever been to Japan or walked past a Timezone (for those of you who live in Perth) you would have seen the "Dance Dance Revolution" coin-op game that is really popular - this is an open source version which is insane. Imagine this on a projector in your in your classroom and all the kids stepping out to the beat- you can hook up an inexpensive playstation mat and have one student leading them all. This is great for fitness, co-ordination and music (moving to a beat).
UltraStarDeluxe - Karaoke (empty orchestra) This is similar to the PS2 Singing program "sing star" - This allows you to create your own karaoke track or download them from the website - sing along and try and mach the pitch and timing of the song. Great for music/singing classes and making them more interactive and involving technology.
The great thing is as these are open source so you can give copies to your students without falling foul of the copyright police.