The Annotator is an open-source JavaScript library and tool that can be added to any webpage to make it annotatable.
Annotations can have comments, tags, users and more. Morever, the Annotator is designed for easy extensibility so its a cinch to add a new feature or behaviour.
The Annotator JS also has a simple but powerful plugin architecture. While the core annotator code does the bare minimum, it is easily extended with plugins that perform such tasks as: serialization, authentication and authorization, prettification etc…
You’ll need to store your annotations data somewhere, but if you can’t then luckily there is an easy way to do this, using AnnotateIt, a hosted web service for storing annotations.