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 project 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 and tagging.

Requirements: CSS3 Support
Demo: http://okfnlabs.org/annotator/demo/
License: MIT, GPL License