iftop - iftop does for network usage what top(1) does for CPU usage.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
ntop - ntopng High-Speed Web-based Traffic Analysis and Flow Collection ntopng is the next generation version of the original ntop, a network traffic probe that monitors network usage. ntopng is based on …
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.
IPtraf-ng - iptraf-ng is a fork of original iptraf-3.0.0.
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.