Nagios is an open source host, service and network monitoring program. Who uses it? Lots of people, including many big companies and organizations. Where can I get it? Right here. Can I get support for it? Yes! If that didn’t satisfy you, click here to get answers to some of your basic questions about Nagios… [Source: Nejc Skoberne – http://nejc.skoberne.net]
http://nagios.org