vnStat - vnStat is a console-based network traffic monitor for Linux and BSD that keeps a log of network...
Zabbix - Track, record, alert and visualize performance and availability of IT resources
iftop - iftop does for network usage what top(1) does for CPU 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.
darkstat - darkstat is a packet sniffer which runs as a background process, captures network traffic...
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.