vnStat is a network traffic monitor that keeps a log of network traffic for the selected interface(s). It uses the network interface statistics provided by the kernel as information source. This means that vnStat won't actually be sniffing any traffic and also ensures light use of system resources.
vnStat is a console based tool which is simple to setup and capable of monitoring your incoming and outgoing bandwidth usage. The default port monitored is eth0, however this may be changed manually. Please consult the Amahi wiki for vnStat
The output allows you to see the bandwidth you've used in a variety of ways, such as by hour, by day, etc. Also, as a bonus it doesn't require the use of root privileges once it has been installed.