"Nagios is a GPL-licensed framework that allows you to intelligently schedule little monitoring programs written in any language you choose. Nagios lets you monitor hosts, services, and networks. Here are a couple of examples of real-world monitoring scenarios.
"In the first example, Company B uses a rather unreliable combination of filters to block unwanted email on its public MXes. The problem is that its business partner, Company A, seems to be particularly disliked by the filters for some reason, so every few weeks, the filters arbitrarily decide to block all email originating from Company A..."
