When you find yourself needing to monitor a particular application, service, or protocol, chances are good that a plugin exists to monitor it. The official Nagios plugins distribution comes with plugins that can be used to monitor a variety of services and protocols. There are also a large number of contributed plugins that can be found in the contrib/ subdirectory of the plugin distribution. The NagiosExchange.org website hosts a number of additional plugins that have been written by users, so check it out when you have a chance.