Arethusa

A backend-independent client-side annotation framework designed to:

Make annotating accessible and enjoyable, lowering the entry barrier for the user by providing access to related resources and services at the point of annotation.
Make the developer's job easier, by giving her tools and a framework for adding new functionality in a modular fashion and for binding diverse resources together in new ways.
Be fully open and have the ability to participate in external workflows that route digital content, together with its provenance, to and from diverse destinations.