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.
vnStat - vnStat is a console-based network traffic monitor for Linux and BSD that keeps a log of network...
Prometheus - An open-source systems monitoring and alerting toolkit.
nload - Monitor network traffic and bandwidth usage in real time