I recently came across the iftop utility, which allows you to view network interface utilization in a top-like display. This can be useful for watching traffic flows on a given interface, or to understand how how much traffic is traversing a given interface. The following picture shows the base iftop display, which contains the SRC and DST IP addresses of the participants, an arrow indicating which way traffic is flowing, and several counters to show traffic sent /received:
