Nagios is a free, open-source monitoring tool that can monitor systems, networks, and infrastructure. It can monitor devices running Linux, Windows, and Unix operating systems, and can monitor critical parameters of application, network, and server resources. For example, Nagios can monitor memory use, disk use, and microprocessor load.

Nagios can send out alerts if critical levels are reached, and alerts users when things go wrong and a second time when the problem has been resolved. Users can choose to work in the command-line interface or select a web-based graphical user interface.