Open-source ticketing tools

These listings include the tag ticketing. 3 projects, led by UVdesk (20k stars), Request Tracker (1.2k stars), Znuny (606 stars).

PHP help desk with email-to-ticket, a support center, and a knowledge base.

Often instead of Zendesk

Enterprise-grade issue and ticket tracking driven by email queues.

Often instead of Zendesk

Web-based ticketing for help desks and IT service management, continuing OTRS Community Edition.

Often instead of Zendesk