Collectl is a system monitoring tool. It logs a wide range of system metrics (disk, cpu, io, etc) and stores them in a variety of formats. It tries to cover all the bases unlike some other tools which focus on one or two methods.
Yet another monitoring app. Copper Egg combines real user monitoring, generic metric logging, server monitoring and maybe other stuff(?) into one application. Pricing starts at $19/month. Not unreasonable.
Munin the monitoring tool surveys all your computers and remembers what it saw. It presents all the information in graphs through a web interface.
Hosted zabbiz service, starts at €5 per server per month. Not sure about data retention.
Nagios® is an Open Source host, service and network monitoring program.
monit is a utility for managing and monitoring, processes, files, directories and devices on a UNIX system. Monit conducts automatic maintenance and repair and can execute meaningful causal actions in error situations.
A howto on setting up monit on an Ubuntu server. Fairly simple and straightforward. Includes basic info on customising alerts and so on.