A nice Spotify app - Boil the frog
This is easily my favourite of the many Spotify apps out there: Boil the frog
Boil the Frog is a Spotify App that will create playlists that gradually take you from one music style to another by analysing adjacent styles and creating an arbitrary path from one to the other.
Sadly not available to the public yet, but the creator of it, Paul, kindly did me one that went from Dubstep to Beethoven. Check it out: Dub-hoven