Cydia - An alternative app store for jailbroken devices.
Magic Playlist - Get the playlist of your dreams based on a song
AppRecs - AppRecs sorts through reviews in the App Store and automatically helps you filter out inauthentic results.
Spotalike - Spotify playlist with similar songs, according to Last.fm
Fnd - Browse Apple App Store and iTunes on the Web
Playlist Machinery - Tools that help you create & organize your Spotify playlists