Diigo, Educational tool or best educational tool

If you've never herd of Diigo.com (as I haddn't before last week) then you should probably go and research it a bit. It's like delicious on steroids and integrated better then any product I've found yet. This has been posted on here because I've created a drupal in education group: http://groups.diigo.com/groups/drupal-in-education

Real quick overview of why this is useful: We all tag things as a site related to drupal in education, we can all go there and check them out. It also can create tag rolls that we might be able to put on our new groups home which would also be kinda cool. This allows you to highlight text and comment on any web page anywhere. You can tag and associate that information to groups (if you choose) or keep the comments private. We're using it internally at the moment to group useful links we find related to drupal / education / web technologies in general. It's also going to allow us to (in trials) leave notes between course developers and faculty, privately embedded within their content and without student's knowledge of what annotations are really just floating out there.