@m well, hypothes.is is open source, but more read-only open source, rather then truly deployable by individuals - you'd have to modify their brwser extensions, etc. Some things are a bit clumsy, but I find it really simple, clever and useful. I already encountered some bugs, but I can live with those. What was a bit harder to crack was how to use it properly on mobile, but later I figure that too. What I find good about it, is that it's stored at a company, but easily downloadable via their APIs using e.g., https://github.com/out-of-cheese-error/gooseberry