Songstack - Swipe to discover new music for Spotify
Join My Playlist - Listen to music live together with your friends in Spotify.
mixmello - mixmello is a free, open source tool to create remixed versions of your favourite playlists.
Parallel - Listen to music with friends over Spotify at the same time
Gnoosic - Even if you don't know what you are looking for - gnod will find it.
TuneMeet - Chat with people who are listening to the same song