From the webpage:
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?