This site by Rory Byrne and Rian Dillon is an amazing use of Spotify's API:
It connects to users' Spotify accounts and records their plays to track top songs, artists, and albums from the community within the past 7 days. User data older than 7 days is then deleted.
I specifically linked to Metro's hip hop feed which tracks users invited on r/hiphopheads but the site also has other feeds and integrates with services beyond just Spotify (via Chome extension).
It's a very cool idea and I'm a huge fan of not retaining more user data than needed.
Check it out and follow their Medium page for updates.