TekSavvy Usage will let you keep track of your current month's bandwidth usage (both on and off peak). This information is usually updated once per day.

Press the select button to refresh the data. You will need to configure your API key for this app to be useful. Follow the link in the configuration page if you do not know it already.

I am not affiliated in any way, shape or form with TekSavvy (beyond the fact they are my ISP). I found a use for this app, and they have OK'd it, so I'm publishing it for others to hopefully find useful as well.