Here is a quote as of this writing taken directly from the Comcast Help & Support page:

"Comcast initiated the 250 GB monthly data usage threshold on October 1, 2008."

So, you see that? I'm lucky enough to not be forced into using their service (yet), however, my parents aren't as lucky. I have been looking since around May at different pieces of software that allow you to monitor your bandwidth to keep below this limit.  Either through the router or by making an educated guess by monitoring the machine being used most on a network.  Granted, you may think this is a lot of usage... but my mom happens to work as a transcriptionist pulling down large files pretty frequently during her work day.

I ran across a Lifehacker article talking about a piece of software called Networx created by a company called Softperfect.  I installed this app and I've been watching it's footprint for a while now.  The actual download is less than 1Mb in size and it seems to be averaging 7Mb of memory usage - pretty good if you ask me!