Bugzilla

Bugzilla is the absolute, unshakeable grandfather of the entire bug tracking industry. Originally developed by the Mozilla Foundation in 1998 (to track bugs for the Netscape browser), it is a completely free, highly complex, open-source platform. While it looks visually terrifying and distinctly like a 1990s web forum, it remains the absolute backbone of massive global open-source projects (including Linux, Apache, and Mozilla itself). Its entire philosophy is "Function over Form." It does not care about beautiful agile roadmaps or sleek drag-and-drop Kanban boards. It cares entirely about the deep, granular, textual data of a bug. It handles highly complex dependency tracking—an engineer can easily flag that "Bug 500" absolutely cannot be fixed until "Bug 492" and "Bug 301" are resolved, preventing chaotic, out-of-order development. It is heavily favored by hardcore system administrators and old-school engineering teams because of its massive "Time-Tested Stability." You can run a Bugzilla server on a cheap piece of hardware in a basement, throw 500,000 massive bug reports at it over 20 years, and the MySQL database will handle the queries flawlessly without ever crashing or charging you a massive per-user subscription fee.