I have amazed by this service ever since I signed up for it a couple of days ago. Basically, Spotify is a free service for streaming a gigantic catalogue of streaming music (supported by the occasional unobtrusive audio ad). The interface looks very similar to that of iTunes and is very easy to work with. Simply search for the album, artist or track you want, and listen away. Playback is so seamless you almost forget your not actually in iTunes listening to local content, and with added features such as “collaborative playlists”, allowing users to build and share playlists in real time, this software is definitely worth a try.
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UPDATE: Today Spotify announced that a small section of their catalogue will now be restricted to certain geographical areas. This is something I have noticed but not to any great extent. Read more here.