MRTG

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.