Zabbix - Track, record, alert and visualize performance and availability of IT resources
Zenoss Core - Zenoss (Zenoss Core) is an open source application, server and network management platform based on...
Observium - Observium
LANState - LANState - Network Mapper and Device Monitor
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
Icinga - Icinga is a fork of Nagios and is backward compatible.