http://cacti.net/
A web based tool to produce graphs based on data stored in RRDTool, usually used as a front end for data collected through systems like nagios.
http://engineering.gomiso.com/2011/01/04/easy-monitoring-of-varnish-with-munin/
Very useful walkthrough on setting up munin to monitor Varnish. Explains how to install and configure munin, which is very simple and very valuable.
