Nagios is a powerful network services monitoring tool. It was previously called NetSaint.
It features a lot of agents to monitor a lot of service types and report it several ways, like SMS, web interface, e-mail, print, …. It is easily extensible by custom-made modules written in a lot of languages.
You can find a lot of screenshots here: http://www.nagios.org/screenshot.php.
This article was first written in June 2003 for the BeezNest technical website (http://glasnost.beeznest.org/articles/32)
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[...] Nagios is of the first kind, Cricket and MRTG are of the second. Both kinds may happily co-exist, and you often need both, if your network is important to you. It is often useful to know when something is going wrong, but also keep a log of those outage and prevent future ones by carefully analysing the usage. [...]