Ticket Tracking Software

www.opensourcehelpdesklist.com - Open Source Help Desk List: A complete list of help desk software products.
en.wikipedia.org - Comparison of issue tracking systems

Candidates

Bugzilla

A Perl based software bug tracking system which can also be used for help desk support. Used by the Mozilla Foundation, creators of FireFox.
Perl and MySQL or PostgreSQL

www.bugzilla.org
landfill.bugzilla.org - Bugzilla Demos
hacksawlabs.com - Install Bugzilla on Ubuntu 8.10
vibhurishi.blogspot.com - Howto install Bugzilla on Ubuntu 9*

OTRS

A trouble ticket system to track telephone calls and e-mails. Designed to allow your support, sales, pre-sales, billing, internal IT, and helpdesk to operate in one system. OTRS is open sourced under the GPL and is written in Perl.
Perl and MySQL, PostgreSQL, Oracle or SQL Server

www.otrs.org
debianadmin.com - How to install OTRS (Open source Ticket Request System) in Debian

RT: Request Tracker

RT is an open source ticket tracking tool. It's primary interface is email with a web back end for administrators. RT has been under development since 1996.
Perl and MySQL, PostgreSQL or Oracle

bestpractical.com/rt/

Trac

Minimalistic web based project management and bug tracking tool. Trac also includes wiki functionality. Written in Python.
Python and SQLite, PostgreSQL or MySQL

trac.edgewall.org
no-names.biz - 40 step to install Apache2, SQLite, Trac, Subversion in Ubuntu 8.04 (Debian)*

ZenTrack

Zentrack is a help desk, bug tracking, and project management system.

www.zentrack.net