Kdevmon is a utility for KDE that monitors the throughput of a network device. It docks in Kicker (the panel of KDE 2) and shows the current network traffic as a diagram. There also is a resizable main window that provides a larger traffic diagram and displays the current net speed in bits per second. Middle-clicking on the dock window or on the main window offers you an overview of the amount of incoming/outgoing bits and the current and the maximum bit rate.
Please report any problems or feature requests to Henning Burchardt <h_burchardt@gmx.de>