Zammad

Zammad is a modern, open-source help desk/ticketing system that can be self-hosted or used as a cloud service. It offers multi-channel support (email, chat, phone, social), a knowledge base, and automation. Its clean interface and active community development make it a compelling alternative to proprietary systems. It targets organizations wanting open-source flexibility without sacrificing a contemporary UX.

It has a free self-hosted tier and paid cloud hosting. It scores decently for being a full-featured, open-source contender that is actively developed and offers a credible path for those avoiding vendor lock-in.