From the webpage:
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.
AnnotateIt is a bookmarklet that claims to allow annotation of arbitrary webpages.
Not what I think anyone was expecting when XLink/XPointer were young but perhaps sufficient unto the day.
I am going to look rather hard at this and it may appear as part of this blog in the near future.
What other features do you think would make this a better topic mapping tool?