Nagios

Nagios is a wonderful tool. I've administered and used it at different jobs for the better part of 5+ years. It's commonly known as the industry standard in IT infrastructure monitoring. In a nutshell-

"Nagios monitors your entire IT infrastructure to ensure systems, applications, services, and business processes are functioning properly. In the event of a failure, Nagios can alert technical staff of the problem, allowing them to begin remediation processes before outages affect business processes, end-users, or customers. With Nagios you'll never be left having to explain why a unseen infrastructure outage hurt your organization's bottom line. "

It's robust, versatile, and effective. These are the traits us SysAdmins look for when needing a tool to get the job done. If you monitor IT infrastructure, Nagios should be a part of your overall system.