Ampache is a web based audio/video streaming application and file manager allowing you to access your music & videos from anywhere, using almost any internet enabled device.
The latest "Web 2.0" site (based on the definition that it does something redundant, but through a browser) blogmusik.net is actually kind of cute. It gives you a virtual ipod and a big library of mp3's you can search, play and create playlists of.