Zabbix - Track, record, alert and visualize performance and availability of IT resources
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.
FastNetMon - FastNetMon is a very fast DDoS analyzer with sflow/netflow/mirror support.
Nfsen - NfSen is a graphical web based front end for the nfdump netflow tools that allows you to:
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
nfsen-ng - nfsen-ng aims to be an in-place replacement for the ageing nfsen.