Zabbix - Track, record, alert and visualize performance and availability of IT resources
nfsen-ng - nfsen-ng aims to be an in-place replacement for the ageing nfsen.
Observium - Observium
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Nagios - Complete monitoring and alerting for servers, switches, applications, and services
NetVizura NetFlow Analyzer - NetFlow Analyzer is a solution for bandwidth monitoring and traffic analysis. It helps with traffic investigation, analysis and reporting