Zabbix
Open Source
Zabbix is enterprise open source monitoring software for networks and applications.
Munin
Open Source
Munin is a networked resource monitoring tool that can help analyze resource trends.
Observium
Freemium
Observium is a low-maintenance auto-discovering network monitoring platform supporting a wide range of device types, platforms and operating systems, etc.
Nagios
Open Source
Nagios known as Nagios Core, is a free and open source computer-software application that monitors systems, networks and infrastructure.
Icinga
Open Source
Icinga is an open source computer system and network monitoring application.
Cacti
Open Source
Cacti is a network graphing solution designed to harness the power of RRDTool's data storage and graphing functionality.