Have you heard about Pandora Free Monitoring System ( Pandora FMS - the Flexible Monitoring System )?
Give it a try. It's a very complete monitoring solution.

Pandora FMS is a monitoring Open Source software. It watches your systems and applications, and allows you to know the status of any element of those systems.

Pandora FMS could detect a network interface down, a defacement in your website, a memory leak in one of your server application, or the movement of any value of the NASDAQ new technology market. Pandora FMS could send out SMS message when your systems fails... or when Google's value drop below US$ 500.