Eventum is a user-friendly and flexible issue tracking system that can be used by a support department to track incoming technical support requests, or by a software development team to quickly organize tasks and bugs.
RT is an enterprise-grade issue tracking system
Support Incident Tracker (or SiT!) is a Free Software/Open Source (GPL) web based application which uses PHP and MySQL for tracking technical support calls/emails
Liberum Help Desk is open sourced under the GPL license and free for use. The help desk software is written in HTML and ASP and is easily modified and customized. All that is required to run Liberum Help Desk is Windows Server running IIS.
SimpleDesk provides a fully integrated ticket based help desk solution, with the capability to have the forum and have full ticket to topic and back functionality, or run in a standalone mode.
GLPI is the Information Resource-Manager with an additional Administration- Interface. You can use it to build up a database with an inventory for your company (computer, software, printers…). It has enhanced functions to make the daily life for the administrators easier, like a job-tracking-system with mail-notification and methods to build a database with basic information about your network-topology.