Diigo
The web it seems is becoming only the surface of what is there. For a while we have had social bookmarking, but I recently found a new tool called Diigo which allows for social annotation. Diigo provides for social tagging as well but what sets it apart is the ability to add annotations to your view of a web site. Simply highlight some text, right click select the item "Add Sticky note" and leave an annotation. Mark the annotation, public, private or share it with a group who will see it if they are logged into Diigo and go to the site where annotations have been left by other group members.This creates a remarkable social space which well facilitates discussions on academic papers, family sites, software and many other things. One of my new favourite tools.