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)