Nagios is an open source monitoring tool that has been available since 1999. It is commonly used to monitor servers, services, and applications. The document discusses how to install and configure Nagios on various platforms like CentOS, Ubuntu, and Mac OS X. It also provides examples of how to monitor common services like HTTP, MySQL, disk space, and custom applications using Nagios plugins. Graphing and alerting capabilities are discussed as well. The presentation concludes with a demonstration and Q&A section.