Redmine

Redmine is the massive, heavily utilized open-source rival to the commercial Atlassian empire. Written entirely in the Ruby on Rails framework, it is a highly flexible, self-hosted web application that provides almost all the core functionality of Jira and Confluence combined, without costing a massive corporate licensing fee. Its signature feature is its "Multi-Project Support." Many simple bug trackers force you to run a separate instance for every project. Redmine allows a single massive server installation to host 500 completely isolated projects. A software agency can manage the bugs for "Client A's App," "Client B's Website," and "Internal HR Tool," each with completely different custom fields, workflows, and user permissions, all from one central dashboard. Like Backlog, it heavily emphasizes holistic project management. It does not just track bugs; it features incredibly robust built-in Time Tracking, native Gantt charts, and a built-in Wiki system. A developer logs their bug fix time directly into the Redmine ticket, and at the end of the month, the agency uses that exact Redmine report to automatically bill the client for the hours worked.