Fiddler - Fiddler is a debugging program for websites.
Magic Playlist - Get the playlist of your dreams based on a song
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
Spotalike - Spotify playlist with similar songs, according to Last.fm
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.
Playlist Machinery - Tools that help you create & organize your Spotify playlists