Hypothesis[archived] is a project that offers an annotation layer on top of the web. It's something that I always wanted to see, and something that has been tried many times in the past. In fact, while signing up for Hypothesis today I discovered I already had an account from 2012!
I installed the unofficial Firefox extension[archived] and decided to give it a try. I like that you can use to annotate PDFs, which is something I've wanted in the past. I don't like that I can't self host my annotations and make them available through a decentralized protocol.