iftop: iftop: display bandwidth usage on an interface
iftop: 
iftop: iftop does for network usage what top(1) does for CPU usage. It listens to network traffic on a named interface and displays a table of current bandwidth usage by pairs of hosts. Handy for answering the question "why is our ADSL link so slow?".
iftop: 
iftop: http://www.ex-parrot.com/pdw/iftop/