The Filter is a neat, free piece of software that helps you build fantastic playlists from the music stored on your Mac. It's like a musical assistant who knows exactly which songs will fit together and which songs won't.
To build a playlist you open up iTunes and let The Filter analyse your library. You then click on 'Create a Playlist' to generate one based on your choice of artist or song. The music in the playlist comes from the music in your collection - it is a great way of rediscovering music you forgot you had!