Till date, following are the most widely used open source and free ticketing system for Linux.
osTicket - The most popular lightweight support ticket system.
eTicket - A PHP-based ticket system with a capable ticket manager
Request Tracker - Mature enterprise-grade issue tracking system used by thousands of organizations.
OTRS - The most successful and long lasting open source system for IT and Help Desk management.
Zentrack - Highly flexible project management, bug tracking and help desk solution
PHD Help Desk - Registry and follow-up ticketing system specially used in Service Desk area.
I hope you will find the best from the above list as per your organization requirements…