This (http://www.paessler.com/prtg) looks useful.
QuotePRTG Traffic Grapher is an easy to use Windows software for monitoring and classifying bandwidth usage. It provides system administrators with live readings and long-term usage trends for their network devices. The most common usage is bandwidth usage monitoring, but you can also monitor many other aspects of your network like memory and CPU utilizations.
Have a look at the 'tour' for more info.
Excellent product since it picks the data up from the router via SNMP. I use this alongside NetMeter. NetMeter on the main PC to give me a quick and dirty view of usage and PRTG for more accurate information.