Nagios is the industry standard in Open Source enterprise-class network monitoring for good reason.

It allows you to gain insight into your network and fix problems before customers know they even exist. It is stable, scalable, supported, and extensible. Most importantly, it works.
Comprehensive Network Monitoring

* Windows
* Linux/Unix
* Routers, Switches, Firewalls
* Printers
* Services
* Applications

Immediate Awareness and Insight

* Receive immediate notifications of problems via email, pager and mobile phone
* Multi-user notification escalation capabilities
* See detailed status information through the Nagios web interface

Problem Remediation

* Acknowledge problems through the web interface
* Automatically restart failed applications, services and hosts with event handlers

Proactive Planning

* Schedule downtime for anticipated host, service, and network upgrades
* Capacity planning capabilites through usage monitoring

Reporting Options

* SLA availability reports
* Alert and notification history reports
* Trending reports through integration with Cacti and RRD-based addons

Multi-Tenant/Multi-User Capabilites

* Multiple users can access the web interface
* Each user can have their own unique, restricted view

Integration With Your Existing Applications

* Trouble ticket systems
* Wikis

Easily Extendable Architecture

* Over 200 community addons are available to enhance Nagios

Stable, Reliable, and Respected Platform

* 10 years in development
* Scales to monitor 100,000+ nodes
* Failover protection capabilities
* Winner of multiple awards
* Constant media coverage

Huge Community

* 250,000+ users worldwide
* Active mailing lists
* Extensive community website network

Customizable Code

* Open Source Software
* Full access to source code
* Released under the GPL license