iCommune

iCommune is a standalone open source application for Mac OS X that extends Apple's iTunes to share your music over a network. You can share the music in your iTunes library and access other iCommune music collections. iCommune music collections appear as playlists in your iTunes window. You can browse through them, and choose to stream or download the music they contain.

The new version of iCommune is finally available for download. It's definitely an alpha version, but it seems to work pretty well for me. Enjoy!