Nagios is a monitoring tool under GPL licence. This tool lets you monitor servers, network hardware (switches, routers, …), applications and services. A lot of plugins are available (more than 3000) and its big community makes Nagios the biggest and the most maintained open source monitoring tool. This tutorial shows how to install Nagios 4.0.1 on CentOS 6.4.
Nagios 4 is the latest stable release published the 20th of last september. This release improved disk I/O latency, some new configuration variables, and other specifications available here : What’s new in Nagios core 4.X.X.