Zammad
Easy to use but powerful open-source support and ticketing system.
Trudesk
Trudesk is an open-source help desk/ticketing solution.
Roundup Issue Tracker
A simple-to-use and -install issue-tracking system with command-line, web, REST, XML-RPC, and e-mail interfaces. Designed with flexibility in mind - not just another bug tracker.
Request Tracker
An enterprise-grade issue tracking system.
OTOBO
Flexible web-based ticketing system used for Customer Service, Help Desk, IT Service Management.
osTicket
Manage, organize and archive all your support requests and responses in one place.
MantisBT
Self hosted bug tracker, fits best for software development.
ITFlow
Client IT Documentation, Ticketing, Invoicing and Accounting Web Application for MSPs (Managed Service Providers).
Iguana
Iguana is an open source issue management system with a kanban board.
GlitchTip
Open source error-tracking app. GlitchTip collects errors reported by your app.
FreeScout
Open source clone of Help Scout: email-based customer support application, help desk and shared mailbox.
Erxes
Marketing, sales, and customer service platform designed to help businesses attract more engaged customers.
Bugzilla
General-purpose bugtracker and testing tool originally developed and used by the Mozilla project.