Zabbix - Track, record, alert and visualize performance and availability of IT resources
Observium - Observium
nfsen-ng - nfsen-ng aims to be an in-place replacement for the ageing nfsen.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Icinga - Icinga is a fork of Nagios and is backward compatible.