1: Bugzilla
Bugzilla is a web-based general-purpose bugtracker and testing tool originally developed and used by the Mozilla project. Bugzilla is written in Perl, and works on various databases including MySQL and Oracle.
2. MantisBT
MantisBT is an open source issue tracker. Users are able to get started in minutes and start managing their projects while collaborating with their teammates and clients effectively. Mantis issue tracking system is written in PHP, and works on various databases including MySQL, MS SQL, PostgreSQL.
3. Redmine
Redmine is a flexible project management web application. It is cross-platform and cross-database and written in Ruby on Rails.
4. Request Tracker
RT is a battle-tested issue tracking system which thousands of organizations use for bug tracking, help desk ticketing, customer service, workflow processes, change management, network operations, youth counselling and RT is written in Perl.
5. Fossil
Fossil is written in C, and uses SQLite database. Fossil has a built-in and easy-to-use web interface that simplifies project tracking and promotes situational awareness. It also integrates the openSearch functionality. Integrates with Subversion.
An open source, multi-platform system for issue tracking and team collaboration. It can be used to store, share and track issues with various attributes, description, comments and file attachments. It is easy to install and use but has many capabilities and is highly customizable.