Bugzilla is the leading open-source/free software bug tracking system, with high-profile installations at mozilla.org, GNOME, Red Hat, and NASA, among others. It features a comprehensive set of fields, attachment management, inter-bug dependencies, email notification of changes, a powerful query interface, reporting, and Web, XML, email, and console interfaces. It can be managed entirely using a Web-based UI. The user UI is customizable using templates.
RT is an industrial-grade trouble ticketing system. It lets a group of people intelligently and efficiently manage requests submitted by a community of users. RT is used by systems administrators, customer support staffs, NOCs, developers, and even marketing departments to track issues, outages, bugs, requests, and all kinds of other things at thousands of sites around the world.
TUTOS (The Ultimate Team Organization Software) is a groupware, ERP (Enterprise Resource Planing), CRM (Customer Relationship Management), and PLM (Project Lifecycle Management) suite that helps small to medium teams manage various things in one place. Its features include personal and group calendars, an address book, product and project management, bug tracking, installation management, test management, scrum management, a task list, notes, files, mailboxes, and useful links between all of the above.
PHP Trouble Ticket provides a centralized means of problem reporting and tracking in multi-user environments (medium to large companies, development groups, etc.). It helps direct problem reports to the responsible administrator(s), keeps track of the status of a problem being worked on, and offers an up-to-date list of recent changes applied. It aims to be widely configurable, easy-to-use even for those not technically inclined, and multi-language capable.
Roundup is an issue-tracking system that is simple to use and simple to install. It has command-line, Web, and email interfaces. It manages a number of issues (with flexible properties such as "description", "priority", and so on) and provides the ability to submit new issues, find and edit existing issues, and discuss issues with other participants. The system facilitates communication among the participants by managing discussions and notifying interested parties when issues are edited.
Ozibug is a platform-independent, Web-based bug tracking system that can be installed and run straight out of the box on any Java servlet capable platform. It can be used to manage the reporting of bugs and tracking of their progress from discovery through to resolution. It provides standard features such as file attachments, multiple module support, email notifications, custom reports, and RSS feeds. For those who require more flexibility, it also offers fine grained access control, database storage, pluggable authentication, an incoming mail gateway, and customizable appearance, email notifications, and reference data.
Bugzero is a Web-based change management and issue tracking system used in a distributed team environment to track software bugs, hardware defects, test cases, or any other issues. It can also be used equally well as a helpdesk customer support, trouble ticketing, or email management system to collect and manage customer feedbacks, incidents, requests, and issues. It is easy to use, but still flexible and adaptive, and can be configured to fit to your organization's unique business process and workflow.