Nagios

Nagios offers complete monitoring and alerting for servers, switches, applications, and services.

Prerequisites
Decide which web server you want to use and set it up:
 * Apache with PHP
 * Lighttpd with PHP

net-analyzer/nagios
Set the right USE flag for :

Also enable the right USE flags for the plugins you want for.

Finally install :

Apache
Enable the Nagios module for Apache:

Restart the Apache service:

Lighttpd
Enable the Nagios config for Lighttpd:

Restart the Lighttpd service:

Permissions
Add the user name(s) to the nagios group, whom are allowed access to the Nagios service:

Once done, completely log-out from all shells and relogin for the update to apply.

Boot service
You can now start Nagios:

To start Nagios at boot time, add it your default runlevel:

Testing
Open your browser and navigate to "http://localhost/nagios".