Sharing music online and getting access to my music collections from various computers has always presented a legal and logistical mess.
That is until YumSugar turned me on to Simplify Media, a site that lets you listen to your home iTunes library over the internet and browse and play your friend's music. It's a free service and works on both PCs and macs.
I love the fact that unlike some of the other sites I've used, you don't have to upload your music and it's legal because your files are streamed only within your private group.
Your group can be made up entirely of your own computers, for example home and office. You can also invite friends to join. Friends cannot burn remote songs to CDs, transfer them to an iPod or mp3 player, or access them when you are logged off.