Web: Your favourite musician probably has many songs you haven't heard or listened to lately. The Long Tail makes a playlist of such rarely played songs by any one artist on Spotify.
The concept is similar to Forgotify, which finds songs that have never been played on Spotify, but that often highlights music you won't like. In The Long Tail's case, you can choose the artist or band. Plus, when you click play, the playlist is automatically opened in your Spotify desktop client, with its many useful features. The Long Tail is a neat and simple app that does what it's supposed to.