"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
"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..."