Bugzilla

Bugzilla is most popular and widely used "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.

Bugzilla is used to

• Track bugs and code changes
• Communicate with teammates
• Submit and review patches
• Manage quality assurance (QA)

Its feature includes:

• Advanced Search Capabilities
• Email Notifications Controlled By User Preferences
• Bug Lists in Multiple Formats (Atom, iCal, etc.)
• Scheduled Reports (Daily, Weekly, Hourly, etc.)by Email
• Reports and Charts
• File/Modify Bugs By Email
• Time Tracking
• Request System
• Private Attachments and Comments
• Automatic Username Completion or Drop-Down User Lists
• Patch Viewer
• "Watch" Other Users
• Move Bugs Between Installs
• Save and Share Searches
• Excellent Security
• Powerful Extension Mechanism for Highly Customizable Installations
• Custom Fields
• Custom Workflow
• Full Unicode Support
• Localization
• mod_perl Support for Excellent Performance
• Webservices (XML-RPC) Interface
• Control Bug Visibility/Editing with Groups
• Impersonate Users
• Multiple Authentication Methods
• Support for Multiple Database Engines
• Sanity Check