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
nload - Monitor network traffic and bandwidth usage in real time
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...
NewRelic - New Relic is a Software Analytics company that makes sense of billions of metrics across millions of apps. We help the people who build modern software understand the stories their data is trying to tell them.