This is the system we use at Red Hat for enterprise support and services contracts.
The system is designed around a simple modular API that allows for dropping new modules right in place without any modification of the existing code. The base API includes many functions that allow module designers to save time and lines of code by using the existing functions built into the Issue Tracker core.
* Modular API
* MySQL & PostgreSQL support
* Unlimited groups
* Group and System Announcements
* Browser based
* Email Interface for pure email interact
* Theme system for easy interface customization
* File uploads
* Support for unauthenticated modules
* Report system for groups and individual users
* Statuses, Categories, and Products editable on per-group basis
* Definable permission set to create roles for users
* Template based for easy UI modifications (Smarty Template Engine)
* Group quotas, either issues or hours based
* Email and SMS alerts
* and much more
Check out the screenshots and demo at http://www.issue-tracker.com, or join the development at
http://sourceforge.net/projects/issue-tracker/“
Link: issue-tracker.com