If you have started KDE, it docks in kpanel and shows the current network traffic as a diagram. There also is a resizable, dockable main window that provides a bigger traffic diagram and displays the current net speed in bits per second. You can adjust the color, scaling, graph style, refresh time, device and style of Kdevmon. Kdevmon is programmed for KDE but also works on other window managers. The only difference is that there is no docked window.
Further suggestions are always welcome.