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