An excellent ncurses based utility to see what goes through your network interfaces.
It shows the number of packages transmitted/received, bytes transmitted/received, currently open connections and their state. It also differentiates between TCP, UDP, ICMP and non-IP traffic.
It is possible to monitor all interfaces at once, or to just see what one interface is doing.
I like to describe it as "tcpdump for dummies" (in the least negative sense of the phrase, of course).
It was written by Gerard Paul Java in 1997, and distributed under the terms of the GPL.