Come October, Comcast will start limiting bandwidth to 250GBs, though it is not likely for many users to break this limit, having a good bandwidth monitor could save you from trouble. Previously mentioned Cyber Bandwidth Monitor does its job pretty well, but lacks in features.
Bitmeter on the other hand is a pretty good tool to monitor your bandwidth, with strong reporting and a easy to use interface.
Bitmeter sits in the system tray and continuously tracks the bandwidth used to upload and download data, hovering over the bitmeter icon shows you basic stats of upload / download speed and the total data that has been uploaded and downloaded for the day.
Along with the basic statistics you can also view detailed statistics about bandwidth usage per hour, per day and per month.
Another interesting thing about Bitmeter is the ability to set ISP restrictions on the amount of data you are allowed to transfer.
You can easily set to be notified if the bandwidth usage has exceeded certain %age, which is pretty good considering the limits Comcast will have in place, the alerts will help you curtail your usage before you cross the limit.
Bitmeter also has a handy tool called Calculator that will show you the amount of time it will take to download a given file, based on your current speed. This can help you schedule downloads at certain time.
Overall Bitmeter looks like a pretty good solution to monitor your bandwidth and you get all of these features absolutely free, so before you start spending to get a expensive bandwidth monitor, give Bitmeter a try and save yourself some money and headaches while you monitor your bandwidth with ease.