PageProof makes it easy to comment and contribute suggestions on a range of materials (from Word docs and web pages to video and other visuals). And it doesn't clog up your inbox. Review collected by and hosted on G2.com.

I enjoy using it so far and can't think of a feature request. Of course, the quality of your interactions there will depend in part on how clear and helpful the comments are. Review collected by and hosted on G2.com.