I got used to 3G data meters from my Android phone. After installing 3G in Windows netbook, I looked for similar applicatio for Windows. Ideally something that just sits in the tray bar and reports current quota usage.
FreeMeter is free (used to be opensource), but aside from messy installation, UAC issues, and crashes, it doesn't do the main job - it doesn't tell me how much of my monthly quota has been consumed so far a how much is left.
NetWorkx does the job although it requires a little configuration.
Firstly I had to set my monthly quota in the quota settings, but I still had the issue that my billing period is not aligned to whole months. It's instead 13th to 12th of the next month. Changing first day of the month to something else than 1st is possible, but the option is hidden in huge settings dialog.
Hovering over NetWorkx tray icon shows current download/upload speed by default. I've changed the configuration to show quota usage instead.
NetWorkx provides some nice reports to analyze where all the bandwidth goes. Unfortunately there's no way to measure connection speed along commute route that I could use to compare providers, but I've found different solution for that.
I should note that I wouldn't have to worry about all this stuff if carriers just provided reasonably priced flatrate services. I bet it's possible for the price they are charging now.