If your concern is http traffic, just install a transparent proxy server e.g squid with squid-log-analyzer or use something like clearos.  

From: skunkworks-bounces@lists.my.co.ke [mailto:skunkworks-bounces@lists.my.co.ke] On Behalf Of Daniel
Sent: Thursday, October 28, 2010 11:03 AM
To: Skunkworks Mailing List
Subject: [Skunkworks] Bandwidth monitoring tool

Hi,
Anyone out there who can recommend a good bandwidth monitoring tool??
Would like to monitor bandwidth utilization on client computers within a LAN.
Preferably if it allows to monitor per IP address.

Rgds,

Daniel