PRTG Network Monitor by Paessler AG is an all-in-one network monitoring platform with 500+ built-in sensor types. The freeware edition supports 100 sensors at no cost.
Founded in 2006 in Ireland, Observium is an auto-discovery network monitoring platform. It automatically identifies and monitors thousands of device types, offering a free Community edition and a paid Professional edition.
LibreNMS is a PHP-based open-source auto-discovery network monitoring system that discovers network devices via SNMP, with 1,600+ device templates and a flexible alert rule engine.
Founded in 2001, Cacti is an open-source RRDTool-based network traffic graphing tool that collects data via SNMP and generates visual charts, with template-based device management and 25 years of trend analysis.