We're expanding our Nagios 3 setup, and are frequently coming across new NRPE or general Nagios plugins to test our existing infrastructure. This is in dribs and drabs though - it would be useful to get a summary of plugins that the Nagios users out there most value.
Please list a single plugin per post, preferably with a short description of why you love it and a link to the MonitoringExchange or plugin developer site. This way folk can vote for plugins already listed and we can see them in preferential order.
It would be better to not list plugins that others have already mentioned, for the same reason. If you have more to add regarding a plugin someone else has listed, please leave a comment to their answer.