[cyanydeez]

i wrote up a white paper a decade ago imagining a wikipedia like 9verlay on websites that would add a meta level of user interaction. just takes some advanced fingerprinting of the annotated content and some social media like subscription and blocking mechanism. it was called _wikilay_

[qwertyzxcvmnbv]

There's also https://web.hypothes.is/

[scroot]

Which I believe is built with annotatorjs [1]. I have used that library on a couple of projects and it's been around for quite a while.

[1] https://github.com/openannotation/annotator/