www.simplifymedia.com
This is a very, very cool app that lets you share your iTunes (or Winamp) music library with up to 30 people, to stream as if you were on the same network.
It works very smoothly with iTunes, and there's an iPhone app that will let you stream your or your friends' pc's music to your iPhone over wifi or 3G. My brother was streaming my library through his car radio (using a cassette adapter) and said it sounded great.
Supposedly a feature like this will be in iTunes 8, which may come out next month, but it's very cool to see this functionality now, especially when it's this easy to install and use.