Automated playlists, smart, if you will
Back when Spotify wasn't really a thing, I used to use iTunes to manage my music library. The music I had I carefully curated, adding metadata such as genres, moods, etc. iTunes made it possible to create smart playlists that would automatically update based on the criteria I set. I loved it.
As Spotify became more capable, I tried switching over, but the lack of smart playlists was a dealbreaker. Spotify also didn't seem to be interested in adding this feature, so I decided to take matters into my own hands. Using the power of the Spotify API, I created a web app that allows you to create smart playlists based on your own criteria. You can set up rules for what songs should be included in the playlist, and the app will automatically update the playlist for you.
What now?
The concept is simple: you can create a playlist with rules that determine which songs should be included. For example, you can create a playlist that includes all songs from a certain artist, or all songs that are tagged with a certain genre. The app will then automatically update the playlist for you, so you don't have to worry about it.
Easier said than done, and although the backend processing was relatively easy (that is, less prone to errors), bugs in the frontend still pop up from time to time. Over time, I have rewritten the frontend three times, but the backend has remained the same.