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.
XPTracker is a radical alternative agile planning tool aimed at keeping out of your way and making sure you can see the stuff you care about at a glance. Included in this tool is a very advanced search system based on Lucene, an easy-to-use planning system based on Queues, and a customizable reporting engine (with burndown/scope creep charts) called Timelines.
Redmine is a flexible project management and issue tracking Web application. It supports multiple projects, flexible role based access control, Gantt charts and calendars, news, document, and file management, feeds and email notifications, per-project wikis and forums, time tracking, custom fields for issues, time entries, projects, and users, SCM integration (SVN, CVS, Git, Mercurial, Bazaar, and Darcs), issue creation via email, multiple LDAP authentication support, user self-registration support, multi-language support, and multiple database support.