What do you mean by best?
What features are you looking for?
What is your comfortable tech expertise to configure, organize, and maintain monitoring tools?
There are a few good OSS monitoring tools out there.
Alerta - Distributed, scaleable and flexible monitoring system.
Canopsis - Opensource Hypervision and Data Aggregation Software
Cacti - Web-based network monitoring and graphing tool.
Cabot - Monitoring and alerts, similar to PagerDuty.
Centreon - IT infrastructure and application monitoring for service performance.
check_mk - Collection of extensions for Nagios.
Flapjack - Monitoring notification routing & event processing system.
Icinga - Fork of Nagios.
LibreNMS - fork of Observium.
Monit - Small Open Source utility for managing and monitoring Unix systems.
Munin - Networked resource monitoring tool.
Naemon - Network monitoring tool based on the Nagios 4 core with performance enhancements and new features.
Nagios - Computer system, network and infrastructure monitoring software application.
Node-Bell - Real-time anomalies detection for periodic time series, metrics monitor.
Observium - SNMP monitoring for servers and networking devices. Runs on linux.
Opsview - Based on Nagios 4, Opsview Core is ideal for small IT and test environments.
Riemann - Flexible and fast events processor allowing complex events/metrics analysis.
Sensu - Open source monitoring framework.
Sentry - Application monitoring, event logging and aggregation.
Serverstats - A simple tool for creating graphs using rrdtool. (source on github)
Seyren - An alerting dashboard for Graphite.
Shinken - Another monitoring framework.
Xymon - Network monitoring inspired by Big Brother.
Zabbix - Enterprise-class software for monitoring of networks and applications.
Zenoss - Application, server, and network management platform based on Zope
What features are you looking for?
What is your comfortable tech expertise to configure, organize, and maintain monitoring tools?
There are a few good OSS monitoring tools out there.
Alerta - Distributed, scaleable and flexible monitoring system.
Canopsis - Opensource Hypervision and Data Aggregation Software
Cacti - Web-based network monitoring and graphing tool.
Cabot - Monitoring and alerts, similar to PagerDuty.
Centreon - IT infrastructure and application monitoring for service performance.
check_mk - Collection of extensions for Nagios.
Flapjack - Monitoring notification routing & event processing system.
Icinga - Fork of Nagios.
LibreNMS - fork of Observium.
Monit - Small Open Source utility for managing and monitoring Unix systems.
Munin - Networked resource monitoring tool.
Naemon - Network monitoring tool based on the Nagios 4 core with performance enhancements and new features.
Nagios - Computer system, network and infrastructure monitoring software application.
Node-Bell - Real-time anomalies detection for periodic time series, metrics monitor.
Observium - SNMP monitoring for servers and networking devices. Runs on linux.
Opsview - Based on Nagios 4, Opsview Core is ideal for small IT and test environments.
Riemann - Flexible and fast events processor allowing complex events/metrics analysis.
Sensu - Open source monitoring framework.
Sentry - Application monitoring, event logging and aggregation.
Serverstats - A simple tool for creating graphs using rrdtool. (source on github)
Seyren - An alerting dashboard for Graphite.
Shinken - Another monitoring framework.
Xymon - Network monitoring inspired by Big Brother.
Zabbix - Enterprise-class software for monitoring of networks and applications.
Zenoss - Application, server, and network management platform based on Zope