Scebo – Customer Support is an advanced WordPress based customer support system by themeskingdom and miljushm.
Set ticket categories, statuses and priorities, notify your customers as soon as you reply to its requests. Install Scebo add-ons to extend its functionality and usability.
Plugin website: http://www.themeskingdom.com/scebo/documentation.php
Download: http://wordpress.org/extend/plugins/scebo/