Magic Playlist - Get the playlist of your dreams based on a song
Rocketgraph - Rocketgraph is an open-source drop in replacement to Firebase, except it's much better.
Playlist Machinery - Tools that help you create & organize your Spotify playlists
Discover on Demand - Just like Spotify's Discover Weekly, but whenever you want.
Spotify.me - Beautiful analytics on your Spotify listening habits 🎧
Steam Database - This tool was made to give better insight into the applications that Steam has in its absolutely huge database.