Diigo is a research tool on steroids. You can write comments in the margins and place moveable sticky notes on any web page, or highlight useful quotes. Diigo collects the pages and your notes, saving them for you to reference at any time. As the teacher, you might collect a set of annotated web pages for a project to present to your class. Diigo allows you to set up your annotated pages as a slideshow. Maybe you have divided the class into research teams. Give each team a Diigo account, and let them set up the permissions for editing and viewing by you and the rest of the class members. I have even used Diigo on published Google Documents, something you might want to consider as you read drafts of your students’ writing [Google Docs does have its own very limited comment feature]. Students can use Diigo comments and notes to assess each other’s writing.
The Diigo creators are highly motivated, constantly updating and innovating this powerful tool. A possible drawback to using it is that you’ll have to download a Diigo toolbar to each PC the students use. If you only access computers through a school lab, you may need to convince the tech people that this is a worthwhile download. The fact that it is free may help.