I run a small colocation with a few friends and needed a way to track whose servers were chewing up our Bandwidth. I needed something a little cheaper than Solarwinds which many ISPs run.
I lucked into Paessler PRTG Traffic Grapher which will figure out the bandwidth in and bandwidth out of most devices that support SNMP. It also can track things like CPU load, disk space, etc.
Great utility. Check it out.