Since my webserver broke down, while I was caught on a ship to Japan, I have relied on the excellent monit to have an eye on all my important services.
Currently their inventors, who give the client-version away for free are working on a remarkable evolution of their M/Monit-tool, a solution to keep track of multiple monit-instances. It only used to give you alarms and show events. Now it will record your system load and memory usage.
If you already have monit installed, this is a great complement. Find out about the beta-version here.