Spotify is an application for Windows, Mac and Linux (via Wine only) that lets you search from a library of online streaming music that you can organise into your own playlists that can then be shared online with other Spotify users. It’s free to use save for the odd audio advert (which are very infrequent) which can be removed either by paying $0.99 for a day pass or £9.99 for a monthly clearance.
I’ve been surprised at how much non mainstream material is available in the library and at how easy it was to install and use on a Linux netbook using only a basic configuration of Wine.
Certainly worth spending some time with.