Nugget Post
I just installed nagios on an Ubuntu Karmic machine, following the excellent quick start guide posted here:
http://www.thedailyadmin.com/2009/03/how-to-install-nagios-on-ubuntu-linux.html
One note though, I didn’t seem to need to add the apache directives for Nagios as described (the “ScriptAlias /nagios/cgi-bin /usr/local/nagios/sbin” part) since the Nagios config file seems to have defined them itself in the /etc/apache2/conf.d/nagios.conf file. Doesn’t really make a difference, just spits out some warnings.
After the install, the Nagios official site has excellent documentation to monitor systems:
David Vassallo's Blog 