sload
Bandwidth Monitoring Tool; Console SNMP traffic monitor
Sload is a console/terminal application which monitors network interface load of the remote device (by SNMP) in real time.
Bandwidth Monitoring Tools For Linux
(Requires Python 2.4 or later)
UPDATE: good known alternative is "ifstat"
http://gael.roualland.free.fr/ifstat/
Example of use:
ifstat -b -i "FastEthernet0/19" -s public@172.18.216.1
It's possible to monitor multiple interfaces in the same time !