Firedoodle allows you to use a web page like a whiteboard, highlighting text, circling important notes etc.. It's great for presentations, jotting down notes and other quick reminders. Firedoodle allows you to doodle on a seperate layer, and save and view them individually. Naturally you can play around with colours and brushes, but you need to be registered. It will seem a little clumsy but will definitly be a great extension, and one of the first I will install when I get a Wacom Tablet or Tablet PC.
Similarily Mystickies allows you to insert small post-it notes on a page, and link them to tags. It works simmilarily in principle, but is neater and less clumsy with less freedom in comparison.