iftop listens to network traffic on a named interface,  or
on  the  first  interface  it can find which looks like an
external interface if none is specified,  and  displays  a
table of current bandwidth usage by pairs of hosts.  iftop
must be run with sufficient  permissions  to  monitor  all
network  traffic  on  the  interface; see pcap(3) for more
information, but on most systems this means that  it  must
be run as root.

WWW: http://www.ex-parrot.com/~pdw/iftop/