Nagios

Plugin-based checks Checks for hosts, services, and network devices run as external plugins, so coverage grows by adding or writing a check rather than changing the core.[1]

Email and pager notifications Contacts receive alerts by email or pager when hosts and services change state, keeping response tied to the event.[1]

Status maps and reports A web-based GUI presents the monitored estate with reports; the official Core-versus-XI comparison lists a basic web interface and basic reports for Core.[1][2]

Automatic event handlers Event handlers fire automatically when monitored events occur, so routine recovery steps run without waiting for a human.[1]

Event Broker extensibility Event Broker modules plug into the core, giving developers a documented route to extend monitoring behavior.[1]

Cross-platform agent The NCPA agent collects metrics from Windows, Linux, and Mac machines, covering hosts the Unix-based core does not run on.[1]

Fifty-plus bundled plugins More than fifty ready-made plugins ship with checks for common services, so standard coverage starts without custom development.[1]

On-premises infrastructure teams get a self-operated monitor for servers, switches, routers, and applications with no license invoice attached.[1][2]

Plugin-oriented operators gain an extension model where new coverage means a new check, backed by decades of community check scripts.[1]

Cost-constrained shops that can administer Linux get enterprise-style alerting and reporting from free GPL software with paid support available.[1]

GUI-first teams will feel the configuration cost, since Nagios is driven by files and definitions rather than visual builders.[1]

Zero-ops buyers cannot get it here, because Core has no hosted edition and every installation is self-run.[1]

Teams expecting modern dashboards may find the CGI web interface dated next to current observability front ends.[1]