is an open-source JavaScript library to easily add Annotation functionality to any webpage. site 
, github
Annotations can have comments, tags, links, users, and more. Annotator is designed for easy extensibility so its a cinch to add a new feature or behaviour. Annotator also fosters an active developer community with contributors from four continents, building 3rd party plugins allowing the annotation of PDFs, EPUBs, videos, images, sound, and more.