http://www.zabbix.com/forum/showthread.php?t=25265
A zabbix monitoring script for varnish ported from cacti.
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://tribily.com/
Hosted zabbiz service, starts at €5 per server per month. Not sure about data retention.
