What is the Redmine system?

Redmine is a free and open-source, web-based project management and issue-tracking tool. It allows users to manage multiple projects and associated subprojects. Redmine's features include tracking of multiple projects, flexible role-based access control, issue tracking system, Gantt chart and calendar, news, documents and files management and many more. 

Redmine is written using the Ruby on Rails framework and is cross-platform and cross-database. As of 2015, it was the most popular open-source project planning tool.

Redmine can be used as a bug tracking system, with the ability to create different types of issues called "trackers". It offers time estimation calculations, subtask creation, Gantt view, time tracking, and more.