Tobi Oetiker's MRTG - The Multi Router Traffic Grapher
What it does
You have a router, you want to know what it does all day long? Then MRTG is for you. It will monitor SNMP network devices and draw pretty pictures showing how much traffic has passed through each interface.
Routers are only the beginning. MRTG is being used to graph all sorts of network devices as well as everything else from weather data to vending machines.
MRTG is written in perl and works on Unix/Linux as well as Windows and even Netware systems. MRTG is free software licensed under the Gnu GPL.
Steve Shipway has written a book on MRTG. If you have been looking for something more than the documentation included with MRTG, this might be just what you have been waiting for. Steve is publishing the book via Lulu, so you can get an actual printed and bound copy. Check out Steve's Website for more information.
If you run into trouble, check out support options.
Sponsorship and Appreciation
You like MRTG? You want to encourage me to spend my time, working on it? You want me to implement a custom feature? Check out the sponsorship page.