vnStat is a network traffic monitor for Linux that keeps a log of daily network traffic for the selected interface(s). vnStat isn't a packet sniffer. The traffic information is analyzed from the /proc -filesystem, so vnStat can be used without root permissions. However at least a 2.2.x kernel is required. I like it a lot, because i have traffic limits with my ISP, which are / month. It's very simple and shows real nice and short overview of my traffic per interface in a simple console.
added to cvs