Extra Admin for Comments is a plugin by Graeme Boy that adds more features to the back-end of WordPress. Specifically, it shows the top ten commentors of your posts from either “All Time”, “1 Week” (the past week) or “1 Month” (the past month), and it also shows all comments that have not yet been replied to (unreplied comments).
Download and info: http://wordpress.org/extend/plugins/extra-admin-for-comments/