Mantis Bug Tracker - Mantis Bug Tracker is a free and open source, web-based bug tracking system.
GitLab - Create, review and deploy code together with GitLab open source git repo management software | GitLab
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.
Redmine - Flexible project management web application
Bugzilla - Bugzilla is a "Defect Tracking System" or "Bug-Tracking System".
osTicket - osTicket is a widely-used open source support ticket system.