I've experienced with quite a few Open Source tools for tracking purposes such as Eventum, Mantis and Bugzilla. All these tools is useful on it's own way, depending on what features you are looking at. I've summarize below on the features for each tools.
Eventum is a user-friendly and flexible issue tracking system that can be used by a support department to track incoming technical support requests, or by a software development team to quickly organize tasks and bugs. Eventum is used by the MySQL AB Technical Support team, and has allowed us to dramatically improve our response times.
Mantis is an easily deployable, web based bugtracker to aid product bug tracking. It requires PHP, MySQL and a web server.
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.