What is it:  Google Sidewiki allows you and others to highlight any section of a web page to either comment on highlight or make a reference point to.  It is free and installs in a flash.

How can I use it in my classroom?  If you want your students to read a passage from a website you can now highlight the section, pose a question in the sidebar and start a discussion from within the page through Google Sidewiki.  All users must be registered so anonymous comments cannot be made.