A simple commenting system for MODx Revolution, complete with a backend management interface, threading support, moderation, notifications, and more.
Note: 1.2.1 and above work only for MODx Revolution 2.0.5 and later.
See official documentation here: http://rtfm.modx.com/display/ADDON/Quip
Install via Package Management, then go to Security - Flush All Sessions. Re-login, and the new security policy will be loaded.