I'm not exactly into music games, but this app for iPhone could help me rediscover the genre. If you know what Guitar Hero is, that it shouldn't be difficult to understand the main concept of Street Orchestra: you have to tap on "music notes" coming down from a "panel" to keep playing a specific song. It's very simple and addictive.
Now, what's so cool about Street Orchestra is that you can gather friends and create a "real" orchestra by mixing multiple instruments and playing the same song all together. The iPhones will sync with each other through a local network, so that you'll be really playing the same notes on different instruments at the same time. You can play with up to 200,000 synced iPhones.
The developers asked the Göteborg Symphony Orchestra to test the app, here's the result:
And here's an "how to" video:
This is just great, and available for free in the App Store.