We use Nagios to monitor all of our servers and the servers we manage. It takes a little time to setup the way you want it but after that it's smooth sailing.

"Nagios is a popular open source computer system and network monitoring application software. It watches hosts and services that you specify, alerting you when things go bad and again when they get better."

Nagios: http://www.nagios.org/
Nagios Plugins: http://nagiosplug.sourceforge.net/
Nagios Documentation: http://www.nagios.org/docs/
The Nagios Book: http://www.nagiosbook.org/html/index.html