GNATS is a portable incident/bug report/help request-tracking system which runs on UNIX-like operating systems. It easily handles thousands of problem reports, has been in wide use since the early 90s, and can do most of its operations over e-mail. Several front end interfaces exist, including command line, emacs, and Tcl/Tk interfaces. There are also a number of Web (CGI) interfaces written in scripting languages like Perl and Python.
TestTrack Pro is a powerful defect tracking/bug tracking application. The application's server component runs on Linux servers and can be accessed by both Web and Windows clients. A full-featured defect management solution, TestTrack Pro includes multiple security configurations, integrated email notification, email importation, XML support, and much more. It easily scales from a single user to hundreds.
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.