WATS is a popular solution. Hundreds of sites are running WATS.
WATS has been reviewed by several WordPress experts :
The sky is the limit on how you can use this plugin to engage current, new or soon to be customers and site visitors.
Judith Kallos, theistudio.com
For those looking for a ticketing system that bolts on easily to WordPress, this plugin looks like it could be what you’re looking for.
Jeff Chandler, Weblogtoolscollection.com
WATS is an interesting plugin as in just a few clicks it allows creation of a community space around a product or a technical support portal through the ticket creation system.
This is the plugin that offers the highest number of options because you can set the support interface in the admin, in the frontend, almost anywhere you want.
One more stepping stone taking further the humungous success of WordPress is WATS – WordPress Advanced Ticketing System. It is perfect plugin for all businesses that want to take their customer relationship indices to a whole new level.