Crocodoc is a cool website where you can upload documents (Word, PDF, and others) and then mark up and comment on the document. The best part of the site in my opinion is that after comments are made you can download the document with the comments. Peer review all done digitally!
You can also use Crocodoc to mark up and review webpages. Simply paste the URL on the Crocodoc website and it creates an image of the page to mark up. Along the right hand side of the page is a page history of each persons comments.