Download Monitor is a bandwidth meter. You can easily monitor your network usages/downloads on hourly/daily/monthly basis. If you often run out of bandwidth then you should try it, it may help you in staying within the usage caps.
It’s a simple app with minimal features, it has no fancy graphs, but it works fine (although app icons look little odd) and very easy to use.
Install Download Monitor in Ubuntu (12.10/12.04)/Linux Mint
It’s available in default package repository, so you can install it from Ubuntu Software Center or just execute the command –
sudo apt-get install download-monitor