Jira - The #1 software development tool used by agile teams. Jira Software is built for every member of your software team to plan, track, and release great software.
jtrac - JTrac is an open source and highly customizable issue-tracking web-application written in Java.
Redmine - Flexible project management web application
Traq - Traq is a PHP powered project manager and issue tracker featuring multiple projects, milestones...
Trac - Trac is a project management and bug/issue tracking system. Provides an interface to Subversion and an integrated wiki.
Bugify - Bugify is a simple PHP issue tracker, designed to offer powerful bug tracking capabilities in an easy to use system.