vnStat is a console-based network traffic monitor for Linux and BSD 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.
This program is open source / GPL’ed and can be installed either as root or as a single user. Better instructions are included in the README. Feel free to send comments by email (remove the spamfilter) or try contacting me from irc (Vergo @ IRCNet).