Just discovered this awesome app for iDevices that let's you host music and join other "parties" to sync music. MusicSessions is free right now from the Apple App Store and you can download it to your iPhone, iPod, or iPad.
It's awesome if:
you're just jamming out you're running with someone want to sing-a-long with friends want to combine all the speakers and rave
The app turns listening to music fun and shareable! I highly recommend it