Ask HN: Is there a service that allows attaching metadata to a web resource?

Say I want to create a Chrome extension to let me tag YouTube videos (with more advanced tags than what the built in playlists can provide). Where do I store these tags? I could of course use YouTube's own "datastructures" such as playlists and the notes for each playlist item, but that quickly gets hairy/hacky. Is there some kind of DBaaS that easily allows me to tie metadata to each video, with a simple REST API?