Some of you may remmeber the big brouhaha about iTunes 4.0 being able to stream over the network and the subsequent removal of that feature.
What's funny is that any OS X user can share their entire music library with the entire world (admittedly, it's not nearly as cool as the automagic playlist sharing), not just with fellow iTuners, with four mouse clicks and one simple command-line instruction (hint: it uses the "ln" command).
You can do the same in Windows 2000/XP, but it's a little more complicated.