Nagios is a service and resource monitoring system, built to help prevent and fix problems in enterprise infrastructure.
Nagios is a powerful monitoring system that enables organizations to identify and resolve IT infrastructure problems before they affect critical business processes.
Source: About Nagios (by Nagios Enterprises, LLC)
[Nagios] watches hosts and services that you specify, alerting you when things go bad and when they get better. Some of its many features include monitoring of network services (SMTP, POP3, HTTP, NNTP, ICMP, etc.), monitoring of host resources (processor load, disk usage, etc.), and contact notifications when service or host problems occur and get resolved (via email, pager, or user-defined method).
Source: Nagios page on Sectools.org (site by Fyodor)