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.
Munin is an open-source server resource monitoring tool from Norway with a Perl-based Master/Node architecture and RRDTool graphing engine. Its plug-and-play plugin mechanism keeps deployment simple for small server clusters.
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.