You've got your music library. Your friends have their music libraries. Wouldn't it be nice if everyone could share? That's the idea behind Simplify Media, a free service that streams your friends' music to your PC--and vice-versa.
All you do is install the Simplify Media client (it's available for Windows, Mac, and Linux), set up an account, then configure the music folders you want to share. (Only DRM-free tunes will work.)
Next up: Invite some friends to share your library, and, of course, to share theirs. If you have no cool friends, that's okay: Simplify Media also lets you stream music from your own PC to another--great if you want to listen to your tunes at work or while traveling.
Speaking of travel, there's also a nifty iPhone app, Simplify Music 2, that lets you stream your songs on the run. It costs $2.99.
Because the service is based on streaming, all involved PCs must be online. In other words, you can listen to your friends' libraries only when they're using their systems. And you can listen to your own library only if you leave your home PC running.
What I find really cool about Simplify Media is the way it integrates with iTunes, Windows Media Player, or Winamp. Other than the client app, which works quietly in the background, there's no special software to install.
I've played around with several similar services over the years, and always seemed to run into some kind of firewall-related trouble. Simplify Media worked like a charm right out of the gate.
You can see the service in action in the following video: