SurplusMeter... If you have a broadband service with a monthly download limit, you may find SurplusMeter comes in handy. It measures the download and upload traffic on your Internet connection and keeps a record of your traffic volume. It gives you all kinds of useful output statistics, like daily allowance, average daily usage, accumulated surplus, and more. SurplusMeter is open-source, and is released under the GNU General Public Licence.
What's new in SurplusMeter
Added support for Ethernet adaptors.
Simplified interface for manually adding/deducting bytes.
Fixed a bug which caused measuring to stop after about 4 GB traffic in a short period.
Fixed an issue which prevented SurplusMeter working on some new Mac models.