trafshow!

Munin is a networked resource monitoring tool that can help analyze resource trends and "what just happened to kill our performance?" problems. It is designed to be very plug and play. A default installation provides a lot of graphs with almost no work.

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. However, in Linux at least a 2.2 series kernel is required.

wavemon is a wireless device monitoring application that allows you to watch signal and noise levels, packet statistics, device configuration and network parameters of your wireless network hardware. It should work (though with varying features) with all devices supported by the Linux wireless kernel extensions by Jean Tourrilhes.

darkstat is a packet sniffer which runs as a background process, captures network traffic, calculates statistics and serves them to a web browser.

Ethstatus is a console-based monitoring utility for displaying statistical data of the ethernet interface on a quantity basis. It is similar to iptraf but is meant to run as a permanent console task to monitor the network load.