For all you Comcast users out there, Comcast has decided to cap your previously unlimited Internet service at 250GB per month. That number is the total of your download AND upload usage. Comcast won't provide their customers with any tool to measure their usage and could care less how you figure it out.
So how can you measure the usage of all your devices??? It's easy if you have a Linksys WRT54GL router (or some versions of the WRT54G and WRT54GS) and the Tomato replacement firmware. It has a great built-in bandwidth monitor and graphs for various time periods (daily, weekly, monthly) so you know exactly how much bandwidth all the devices behind your router are using.