Howtoforge posted a guide about installing Nagios on Ubuntu 10.04

This tutorial shows how to set up Ubuntu Lucid Lynx (10.4) server with the Nagios Monitoring System. Nagios is a powerful, highly configurable monitoring and alarming system, which can monitor a wide variety of systems (network, server, daemons, applications). Monitoring could be done for instance for availability or utilization. The monitoring could be restricted to services which are connectable from the outside (e.g. a webserver on port 80/tcp), or with the help of NRPE (Nagios Remote Plugin Executor) plugins for testing could also be executed remote. We will be installing Nagios, Nagios Plugins, and Postfix with this tutorial. Postfix will be configured to send email via your alternate email server. This tutorial will only give you the basic configuration with Postfix, any additional settings can be found in multiple places on the Internet.

  Nagios Installation On Ubuntu 10.04 (Lucid Lynx)