Everybody is talking about Google Toolbar with Sidewiki. Google Sidewiki is a new feature of the Google Toolbar that lets you leave comments about any website. When some else running Sidewiki views that web page, they'll see your comments.
Check it out: http://www.google.com/sidewiki/intl/en/install.html
Youtube video: http://www.youtube.com/watch?v=CsjJOsx84MA