Although it hasn't exactly exploded into the mainstream music-listening population, the music-sharing application Simplify Media has a growing network of fans--in no small part because it's available for Windows, Mac, and soon the iPhone. When you install it, you must choose to sync with either Winamp or iTunes playlists, although you can change your preferred program after installation. After setup and restarting your music player, you'll see a "Simplify Media" selection in your left-hand navigation. Expand it, and you can browse all of the music available from you and your friends.
Claims of resource hogging and installation headaches are somewhat valid, and running it on Windows will likely require the included Apple Bonjour. Running Simplify Media, Winamp, and Simplify Media Peer, the process that streams media from your friends or remote computers, together uses more than a fair chunk of RAM.
Make no mistake with the minor quibbles: Simplify Media is an excellent and simple method of sharing music collections with friends or yourself in remote locations. We would like to see future versions become even more stable, as well as adding compatibility with other jukebox players.