I found this howto for installing Nagios on Linode and thought I'd share it after using it to successfully install and configure Nagios.
Author: Amanda Folson
Published: May 26, 2010
Nagios is a flexible monitoring system framework that makes it possible to maintain consistent monitoring of a broad range of infrastructural components. Many users use Nagios to monitor a broad range of network services over protocols that include SMTP and POP3 (email), HTTP (web), ICMP (ping), and SSH. In addition to simple uptime monitoring, Nagios also contains features for advanced notification and graphing as well as an extensible plugin architecture that allows users to develop simple plugins for accessing unique monitoring information and providing customized reports.