Nagios is a program that will monitor hosts and services on your
network, and to email or page you when a problem arises or is
resolved. Nagios runs on a unix server as a background or daemon
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. This package
contains additional SNMP check plugins from http://nagios.manubulon.com/