Those of you who use Firefox may want to take a look at a new add-on, Firedoodle. It provides a toolbar that lets you annotate any web page — that is, treat that page as though it were a whiteboard — and then save the annotations. You can draw in various colours of variable opacity; you can frame a portion of the page (see the linked thumbnail pics below); and you should be able to place markers at chosen points in the page and leave sticky notes on the page (though I’ve had trouble getting the former to work).
I see this as useful in presentations, when you want to be able to draw your audience’s attention to aspects of a page and to have all of it canned and ready to roll with just your browser. Evidently a form of sharing annotations is being worked on, which might make this add-on even more useful.