Bugzilla!

Trusted by 11,000+ businesses, JIRA is the leading bug tracking, issue tracking and project management tool. JIRA combines issue tracking, agile project management, customisable workflow, and a pluggable integration framework to increase the velocity of your software development team.

Phabricator is a collection of open source web applications that help software companies build better software.

OpenProject is an open source software for project management with a wide set of features and plugins and an active international community. It supports project timelines / Gantt charts, issue tracking, wiki, document management, time and cost reporting, code management, Scrum and much more.

osTicket is a widely-used open source support ticket system. It seamlessly integrates inquiries created via email, phone and web-based forms into a simple easy-to-use multi-user web interface. Manage, organize and archive all your support requests and responses in one place while providing your customers with accountability and responsiveness they deserve.

Mantis Bug Tracker is a free and open source, web-based bug tracking system. It is written in the PHP scripting language and works with MySQL, MS SQL, and PostgreSQL databases and a webserver. MantisBT can be installed on Windows, Linux, Mac OS, OS/2, and others. Almost any web browser should be able to function as a client. Moreover MantisHub is available as commercial hosted solution of MantisBT.

Team Clerk is project and task management software that automatically keeps your team organized and monitors progress of all of your projects and tasks. Let us organize what you do,so you can focus on why you do it.

Tracker is a story-based project planning tool from Pivotal Labs that allows teams to collaborate and react to real-world changes instantly. Its based on agile software methods, but can be used on a wide range of projects.

The Bug Genie is an issue tracking and project management, completely free for small teams.

PMRobot combines Bug Tracking + Client Collaboration + Agile Project Management Software.

RT is an enterprise-grade ticketing system which enables a group of people to intelligently and efficiently manage tasks, issues, and requests submitted by a community of users.

Flyspray is a lightweight, web-based bug tracking system written in PHP for assisting with software development and project managements. Originally developed for the Psi Jabber client project it has been made available to everyone under the LGPL 2.1 licence. Flyspray aims to cut out the unnecessary complexity of other bug trackers focusing on a very intuitive design making it very easy to effectively manage projects.