A lot of my friends and co-workers, as well as myself, are using an iTunes plugin that puts the current song you're listening to in your AIM status message. It's called iChat Status and it's a fun way to see what your friends are listening to at any moment.
Oh sure, I have to deal with "Andre, what the hell are you listening to Ben Lee for!?" or "Can you zip up that entire Wu-Tang album for me and upload it to your server?" But I like sharing music and turning people on to stuff, so it's cool. (btw, Ben Lee's "Grandpaw Would" is the only album worth listening to)
Anyway, what I'd like to be able to do is point my iTunes at my buddy list and when it notices someone playing a song I own, I'd like it to queue up that song to play when it gets a chance.