Songstack - Swipe to discover new music for Spotify
FIEND.fm - "Hot or Not" for music discovery: 2 hrs of tracks in 2 mins
BandNext - Discover bands that sound similar to artists you already love with BandNext. A single click saves your results to a Youtube playlist.
Pacemaker - Create and share mixtapes with friends
22tracks - DJ curated Spotify playlists on iOS
Boil the Frog - Create a seamless playlist between any two artists