Songstack - Swipe to discover new music for Spotify
Magic Playlist - Get the playlist of your dreams based on a song
mixmello - mixmello is a free, open source tool to create remixed versions of your favourite playlists.
Spotalike - Spotify playlist with similar songs, according to Last.fm
Gnoosic - Even if you don't know what you are looking for - gnod will find it.
Spotify - Map shows when two people play same song at same time