Redmine is a flexible project management web application. Written using Ruby on Rails framework, it is cross-platform and cross-database.
Trac is an enhanced wiki and issue tracking system for software development projects. Trac uses a minimalistic approach to web-based software project management. 
Bugzilla is a “Defect Tracking System” or “Bug-Tracking System”. Defect Tracking Systems allow individual or groups of developers to keep track of outstanding bugs in their product effectively. Most commercial defect-tracking software vendors charge enormous licensing fees. Despite being “free”, Bugzilla has many features its expensive counterparts lack. Consequently, Bugzilla has quickly become a favorite of hundreds of organizations across the globe.