Host/service/network monitoring program
http://www.nagios.org/

Nagios is a program that will monitor hosts and services on your network. It has the ability to email or page you when a problem arises and when a problem is resolved. Nagios is written in C and is designed to run under Linux (and some other *NIX variants) as a background process, intermittently running checks on various services that you specify.

The actual service checks are performed by separate "plugin" programs which return the status of the checks to Nagios. The plugins are available at http://sourceforge.net/projects/nagiosplug