vnStat is a console-based network traffic monitor that 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.
Traffic statistics are stored on a hourly level for the last 24 hours, on a daily level for the last 30 days and on a monthly level for the last 12 months. Total seen traffic and a top 10 days listing is also provided.
Frequently Asked Questions: http://humdi.net/vnstat/FAQ