jtrac - JTrac is an open source and highly customizable issue-tracking web-application written in Java.
Traq - Traq is a PHP powered project manager and issue tracker featuring multiple projects, milestones...
Bugify - Bugify is a simple PHP issue tracker, designed to offer powerful bug tracking capabilities in an easy to use system.
Pivotal Tracker - Pivotal Tracker: The awesome, lightweight, agile project management tool for software teams. Get your 30-day Free Trial started today!
asitrack – Assisted Issue Tracking - Assisted Issue Tracking is an inclusive bug and issue tracking software that comes with a lot of customization and provides you with a simple way to add custom fields and dropdowns.
Linear - Streamlined issue tracking for software teams