I have long been a fan of Dropbox, a free online service that allows you to sync files between computers. If I make a change to a document at home and save it to my Dropbox folder when I get to school the next day it is already there and waiting for me. It turns out that the folks over at LifeHacker figured out a way to make iTunes use Dropbox for storing its music as well as the information needed to sync your Apple devices. Setting up Dropbox to store the music is easy, but setting up the syncing thing is a lot more complicated so it is a good thing that the LifeHacker article goes into great detail on it.
In any case, to try this trick out (even if it is just to share your music only) you have to download and install Dropbox on your Macs and/or PCs. Get it up and running and then move your iTunes music folder using the instructions found in the previously mentioned article. If you have a large collection of music you may have to buy a subscription, but keeping up to 2 gigs of files in Dropbox is free. Try it out! As a Mac person that has to live in a Windows world I have to say that this is awesome!