Subsonic is an open source web-based media streamer application that helps sharing your music or listening it anytime/anywhere.
It allows streaming to multiple users & can handle very large music collections.
It is compatible with every audio or video format that can stream over HTTP, like AAC and OGG. Also, Subsonic works with any network-enabled media player, such as Winamp, iTunes, XMMS, VLC, MusicMatch and Windows Media Player.
It can integrate with various web services like Last.fm, Amazon (for cover art) & more.
And, considering media files are generally huge, it supports automatic zipping/unzipping while downloading & uploading them.