bandwidthd is a free and open-source bandwidth monitoring. BandwidthD tracks bandwidth usage and provides graphs

bandwidthd

systemd (RHEL, Debian, Ubuntu, Arch, openSUSE)

darkstat is a free and open-source network statistics. darkstat captures network traffic and provides statistics

darkstat

iftop is a free and open-source bandwidth usage. iftop displays bandwidth usage on interface by host

mrtg is a free and open-source traffic grapher. MRTG provides tool to monitor traffic load on network links

netflow is a free and open-source flow collection. nfdump provides NetFlow collection and processing

nethogs is a free and open-source bandwidth monitor. NetHogs groups bandwidth by process

vnstat is a free and open-source network monitor. vnStat provides network traffic monitor with statistics