I want this as well. You can get the javascript from
http://www.tynt.com/ (it's free) but I don't understand how to add it to the "page template" as mentioned in the video on their site.
A link on how to install it
http://www.tynt.com/support/how-to-i...#axzz1wrgxsybp
Edit: I added the script they provided (after signing up) to the bottom of my headinclude template and it works