From the project website:
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 accompanying site AnnotateIt is a hosted web service for storing annotations.
The Annotator is being developed by the Open Knowledge Foundation.
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