Nagios is the shiznit..

Nagios is a powerful monitoring system that enables organizations
to identify and resolve IT infrastructure problems before they affect
critical business processes.
Nagios gives you the peace of mind that comes from knowing your 

organization's business processes won't be affected by unknown outages.

Nagios is a powerful tool that provides you with:
instant awareness of your organization'smission-critical IT infrastructure.
Nagios allows you to detect and repair problems and mitigate future issues
before they affect end-users and customers.

What Nagios Provides:

By using Nagios, you can:

Plan for infrastructure upgrades before outdated systems cause failures

Respond to issues at the first sign of a problem

Automatically fix problems when they are detected

Coordinate technical team responses

Ensure your organization's SLAs are being met

Ensure IT infrastructure outages have a minimal effect on your organization's bottom line

Monitor your entire infrastructure and business processes

How It Works

Monitoring

IT staff configure Nagios to monitor critical IT infrastructure components,

 including system metrics, network protocols, applications, services, servers,

and network infrastructure.

Alerting

Nagios sends alerts when critical infrastructure components fail and recover,

 providing administrators with notice of important events. Alerts can be 

delivered via email, SMS, or custom script.

Response

IT staff can acknowledge alerts and begin resolving outages and

 investigating security alerts immediately. Alerts can be escalated 

to different groups if alerts are not acknowledged in a timely manner.

Reporting

Reports provide a historical record of outages, events, notifications, 

and alert response for later review. Availability reports help ensure 

your SLAs are being met.

Maintenance

Scheduled downtime prevents alerts during scheduled maintenance 

and upgrade windows.