ntop. 
www.us.ntop.org. or alternatively, apt-get install ntop.
it's pretty, it's functional, it can log stuff to databases, it has lots of
nifty graphs on its web interface.

very good thing to show to management. they'll be impressed by the pretty
GUI, and it's pretty easy to figure out the graphs. (and we all know how
much managers like graphs).

caveat: it's also rather buggy (just ask Bob about his source code audit of
it) and if you're going to run it on a public network, it might not hurt to
chroot it.

Carl Soderstrom.
-- 
Network Engineer
Real-Time Enterprises
www.real-time.com