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.
A great discussion on the comparison between nagios and zabbix and other monitoring platforms. Very useful insights although a few years old.
Hosted zabbiz service, starts at €5 per server per month. Not sure about data retention.
Interesting article on how webpop built their infrastructure on Rackspace's cloud. Includes links to a few useful tools (chef, wormly, new relic, nagios, ganglia, and others).
Nagios® is an Open Source host, service and network monitoring program.