Nagios, a pioneer in IT monitoring since 1999, remains a reliable choice due to its plugin-based architecture and robust notification system. While highly customizable, it has an outdated UI and requires manual configuration. Competitors like Zabbix offer better scalability and a modern interface, while Prometheus excels in cloud-native monitoring. Choosing the right tool depends on an organization’s needs—Nagios for stability, Zabbix for enterprise scalability, and Prometheus for dynamic cloud environments.