Munin
From Freephile Wiki
Munin is a network monitoring system that will provide visual graphs which aid you in seeing resource utilization in your infrastructure.
They are redoing their docs, and the "Munin Guide" is the new stuff http://guide.munin-monitoring.org/en/latest/installation/configuration.html
See Also
Many are based on RRDTool and/or the original wp:Nagios