"Full-featured traffic analyzers for Linux systems such as ntop
and vnstat are widely available, but sometimes you just want a
simple program that gives you fast, basic information about the
amount of traffic going in and out of the hosts on your network.
Darkstat, a packet sniffer that runs as a background process, fills
that role. It gathers statistics about network usage and displays
them over HTTP.
"I tested the latest version (darkstat-3.0.707.tar.bz2) on
CentOS 5 and Fedora 7 systems. You can untar this file with
tar-xvjf darkstat-3.0.707.tar.bz2, then install it
with the usual ./configure; make;