Check out M. Bilal's howto on Bandwidth Monitoring Free & Paid Tools Software Details.
http://community.spiceworks.com/how_to/show/2532
Two useful, free products from Solarwinds:
Real-Time Bandwidth Monitor
http://www.solarwinds.com/products/freetools/real-time-bandwidth-monitor.aspx
Real-Time Netflow Analyzer
http://www.solarwinds.com/products/freetools/netflow-analyzer.aspx
Bandwidth monitoring works well to watch specific interfaces on routers (the WAN router might be interesting, for example). When it comes down to tracking what devices are conversing with local and internet locations, Netflow is the correct tool. The free tools work well as long as you don't need historical data. If you need to track down what happened last night, then the paid versions are what you're looking for.