Revizr lets you upload a document, which then can be “marked-up” by others (each person’s comments appear in a different color). It’s a pretty easy-to-use application, so I’m adding it to The Best Online Tools For Collaboration — NOT In Real Time.
You can’t upload webpages easily now, though they plan on adding that feature in the future. That ability would be particularly helpful with students. When they do indeed upgrade the site to include annotating webpages, I’ll look forward to adding the site to The Best Applications For Annotating Websites .