Notate is a web-based tool for annotating web pages. You can create and display notes, tag your comments and questions, highlight text, attach additional images and documents, and make links to show connections between ideas.  You can also invite others to share in the discussion.   Notate seems ideal for journal groups where students and faculty collectively discuss articles.  For example, this PLOS Featured Image could be used as a springboard for discussion, as could the article it came from:

Users sign up for an account and download (IE 7) or drag (Firefox) 2 buttons to their bookmarks (no Safari, so far).  To begin using Notate, you navigate to a web page you want to annotate and click on the Snap button to capture the page.  Then the Notate 2.1 dialog box comes up and you can add comments, questions, replies, tags, highlighting and additional documents. Notate also allows you to create wiki-like pages where you can view all your snapshot pages, notes and tags, and to which you can add additional subpages.

If you want to check out Notate without registering for an account, try Textensor’s Sandbox area.