Featured Comment Widget is a WordPress plugin by jennschiffer that gives you the ability to shine a spotlight on some of your favorite comments on the site.
Installing this plugin adds the Featured Comment Widget to your list of available widgets in the ‘Widgets’ submenu (which you can find under ‘Appearance’ in your WordPress dashboard). Every comment has an numeric ID, and putting that ID in the widget form results in the that comment, along with the name and Gravatar of its commenter, showing in the sidebar. The Gravatar and username is automatically a link to that comment in its respective post.
Download and info: http://wordpress.org/extend/plugins/featured-comment-widget/