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.
Hosted zabbiz service, starts at €5 per server per month. Not sure about data retention.
A howto on setting up monit on an Ubuntu server. Fairly simple and straightforward. Includes basic info on customising alerts and so on.
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.
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.
Nagios® is an Open Source host, service and network monitoring program.
Munin the monitoring tool surveys all your computers and remembers what it saw. It presents all the information in graphs through a web interface.