Icinga 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. Icinga 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 Icinga. The plugins are
available at http://sourceforge.net/projects/nagiosplug