From Hoo Technologies:
Net Meter monitors network traffic through all network connections on the computer it's installed on, and displays real-time graphical and numerical downloading and uploading speeds. The software supports to display transfer rates of multiple network connections at the same time. It also logs network traffic and provides daily, weekly, monthly, and summary traffic reports. And start days of week and month are customizable. The program allows you to setup a notification to get an alert when you exceed a certain amount of bandwidth usage. And, traffic stopwatch enables you to test bandwidth speed of connections. You can also record transfer rates of connections. Net Meter works with the majority network connections including phone modems, DSL, cable modem, LAN, satellite, wireless, and VPN.
This worked, and did what I needed. It let me know how much data I used over the course of a month.
I can't get rid of the darn thing! It loads and gives me an error message when I turn my computer on, but I can't find the program files anywhere, I can't find a system service it runs, I can't find it in the startup list, and I can't remove it! Google hasn't been much of a help with this problem either.
Get it if you really need a free net meter for a month, but keep in mind that you may not have an easy way of removing it.