Internally we use RT (enterprise-grade ticketing system) for customer support and it is one of the best GPL software around. It enables a group of people to intelligently and efficiently manage tasks, issues, and requests submitted by a community of users. RT manages key tasks such as the identification, prioritization, assignment, resolution and notification required by enterprise-critical applications including project management, help desk, NOC ticketing, CRM and software development.
Linux magazine has published a nice article about RT installation and configuration:
Managing bugs and help requests isn't easy, but it's crucial for effective project management. Using the wrong tool can set your project back to the dark ages, but the right tool can help your team excel. With that in mind, let's look at Request Tracker, an enterprise-grade (and free software) ticketing system written in Perl.
A ticketing system is a piece of software in which every bug, request, or problem is entered as a 'ticket,' which can then be tracked. It can be allocated to someone to fix, given a priority, placed in a queue (to separate out different types of bug or request), commented on, replied to, and finally closed when resolved. The system can also send progress updates and reminders to the initial requester and to other people involved with the ticket. RT handles all of these functions and more.