Prometheus - An open-source systems monitoring and alerting toolkit.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
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
StatsD - Simple daemon for easy stats aggregation.
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.