Stream your iTunes Library to your iPhone, or your other computers, or to other users. Works just beautifully over WiFi, but you need the patience of a saint to use it over 3G. Very nice interface on the iPhone. One flaw: it took over an hour to scan my library, and when it had finished, only managed to find 10k of my 15k songs. (Update: it seems to be slowly adding more songs - up to over 11k now, so I'm guessing it'll have the lot eventually).