osTicket is a widely-used open source support ticket system. It seamlessly integrates inquiries created via email, phone and web-based forms into a simple easy-to-use multi-user web interface. (GPL)
Cross-platform
OTRS is an Open source Ticket Request System (also well known as trouble ticket system) with many features to manage customer telephone calls and e-mails. (GPL)
Linux - FreeBSD - Windows - macOS
UVdesk Helpdesk is an open source ticketing systems with the following functionality: email piping, knowledgebase, modern SLA, automated rules, saved replies, agent privileges, live reporting, analytics, ticket types and unlimited support agents. (MIT)
Cross-platform
3 applications