Nagios is an open source program that can be installed on a server to check on other servers. For instance, you could have it check that sites are up, services are running, an machines are pinging. And if it doesn’t find those things, it can send an email or an SMS so you know about it.
Macworld just posted a great article on this Nagios called, “Installing Nagios on Mac OS X 10.5 Server.”
They also pointed to a nice community where plug-ins are shared called Nagios Exchange.
You can download Nagios here.