. Freeware, runs from the system tray, fully customizable.
Let it load at startup and it records daily, weekly and monthly data transfers and compiles it into a report. You can also set it to alert you when you're approaching your capped Gb/month amount.
EDIT: If you're in the UK try NTL's new 10mb broadband package
which has a 75gig monthly allowance. I'm on their 2.5mb package and their service is excellent, only lost the connection once in 2 years, and that was just a few hours for maintenance.