Look into Networx. It's a free network monitoring system that can be configured to show how much data each user is using on each machine or it can be configured to report on data usage on all machines. It shows which apps are using the data, how much they are using and what time they are using it.
Read about it here:
ClickyBelow is a comment left on the Cnet site: Summary
I've got 3g internet and have a bandwidth limit of 12GB.
I've been recently going over my limit and didn't understand why. (I wasn't using the internet any different than I have in the past.) I did some research and found this program.
I LOVE it!
I'm able to monitor my usage and was able to figure out which programs were hogging my bandwidth.
It turns out my anti-virus (Avast) had added a bunch of monitoring that I didn't need and was using more than 2X what I was using with browsers alone.
Plus, I was able to figure out that I needed to limit my browsers' settings to not auto load videos and scripts.
I'm now able to stay way under my limit again! :)