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.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Paessler PRTG - An advanced network monitoring solution to monitor network up/downtime, traffic and usage.
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.
Observium - Observium
Cacti - Cacti is a complete network graphing solution. It provides a fast poller, advanced graph templating, multiple data acquisition methods, and user management features.