Check out ourTunes, a slick hack for iTunes that allows you to access music in iTunes folders on anyone's computer, assuming their computer is on the same network as you. *read* College Campuses, Large Corporations, etc.
Basically, if your network consists of home, you'll only get other computers within your home. So, in this scenario, not too exciting. However, get plugged to a network with many computers, like most at work, school, etc, and you now have wonderful access to everyone else's music, assuming they, too, have ourTunes.
The hack was created/programmed by a bunch of college students with an opensource mentality. So, customer support is virtually nill, updates are being constantly created to fix bugs, and there's probably not much of a guarantee on the performance.
But, then again, we all seemed to get over the issues wtih other open source, file-sharing, peer-to-peer programs.
Click here to visit the ourTunes site, along with downloads.