Freshly baked by the friendly team here at CakeDC is the Comments plugin.
The comments plugin allows you to enable comments on any controller for any existing model in you application. Built in a manner to allow complete separation from your application, enabling and including the comments functionality is almost too easy.
A good use case is the addition of comments to blog posts. In this case you can facilitate user feedback on information posted on your web site to further enhance the facilities of your existing application.
The documentation takes you through a practical example of how you can include this into an existing application with only a couple of code lines.