Songstack - Swipe to discover new music for Spotify
Momixa - Pick two songs and get the perfect mix
BandNext - Discover bands that sound similar to artists you already love with BandNext. A single click saves your results to a Youtube playlist.
Music Suggestions Ninja - Enter an artist's name & find similar, lesser known artists
22tracks - DJ curated Spotify playlists on iOS
Boil the Frog - Create a seamless playlist between any two artists