The Multi Router Traffic Grapher (MRTG) is a tool to monitor the traffic
load on network-links. MRTG generates HTML pages containing PNG images which
provide a LIVE visual representation of this traffic. Check

WWW: http://www.mrtg.org/

for an example. MRTG is based on Perl and C and works under UNIX and Windows
NT.

MRTG is being successfully used on many sites around the net. 
Check the MRTG-Site-Map, which is at:
http://ee-staff.ethz.ch/~oetiker/webtools/mrtg/users.html