Unfortunately, a transparent proxy like Smoothwall or a traffic monitor like ntop may be your only choices. These will have to sit at the edge of your network or between the edge router and the LAN switch. You can find bootable CD’s with tools like NTOP on it to try this out. See Network Security Toolkit for an example.
You might want to check out WireShark, NetworkProbe, WiFiHopper, MRTG. These tools are able to tell
you all sorts of information about packets being sent, and turn the information into a graphical form. NetworkProbe is done through a web-browser (one of my favorite).