Nagios known as Nagios Core, is a free and open source computer-software application that monitors systems, networks and infrastructure. Developed by Nagios Enterprises
License: Open Source
Nagios is known for its robustness and flexibility, making it suitable for smaller to medium-sized environments, while Zabbix offers a more modern interface and rich visualization tools, making it better suited for larger environments. Both tools excel in real-time monitoring and alerting but differ in terms of user experience and scalability.