Using SimplifyMedia will allow you to add 30 of your friends' iTunes music libraries to your own and access them no matter where they are physically located. Right now the beta software is a free download and only works on PCs and Macs. If you do decide to give SimplifyMedia a whirl, be warned—the software sounds a little sluggish right now and includes one unforgivable pop-up.
A user-repulsing "Buy Music" ad appears while the program is open and can't be closed out, though it can be minimized. If you can look around it, SimplifyMedia sounds like it does what it set out to do, even in its beta state. Oh, no Vista. Solly.
Update: The SimplifyMedia team let us know that the aforementioned offending pop-up has been removed from the program. When the Amazon purchase link was determined to be an annoyance, it got cut. We're all for a team that puts the user first. Sounds like an app to keep an eye on. [Symplify Media via tuaw.com]