Network monitoring software

[Aaron Holmes]

My company has asked me to setup some network monitoring software so we 
can watch what clients are doing on the Internet and so forth.
I know of various windows based packages that do this (such as spector) 
but i dont know of any freebsd or unix based packages.
do any of you have any suggestions? (im /not/ looking for something like 
nagios and big brother, unless nagios can do what i want, of course :))

-Aaron