you've got a broadband connection at home, but your provider puts a cap on your monthly download allotment. That's all
well and good until it's the end of the month and you find yourself strapped (you torrent addict, you). Wouldn't it be
nice to get a warning before everything goes kaput?
Check out SurplusMeter. It simply monitors all of your traffic (both up and down) and reports on how much you're using. You can view a daily average and more. Note that SurplusMeter only monitors traffic, not content, and doesn't "phone home" in any way.
SurplusMeter is free, and you can get the source code here.