I know the guy that made this, so this is vaguely JAG-ish, but it seems pretty neat to me.
Basically if you're at a party (or just at someone's house) and they're playing music that's scrobbling, you can enter their username here and the music will then scrobble to your account as well.
- nice to have record of stuff you've listened to with other people
- if something cool was playing but you didn't know what it was, you can find out, I guess
This is also good if, like me, you resent ever listening to music that isn't contributing to your last.fm profile (it's a problem).