The app formerly known as Polyjamer
You can make awesome music with it..
Start a band! get everyone you know to install the app, connect to the same WIFI network or HOTSPOT, and all devices will sync together! Dive in to epic jam sessions anywhere. you can even make some quality noize with 20+ devices! (highly recommended).
You must be within 2 - 3 meters of other humans to use this feature.