One of the problems with Nagios is that initial set up & configuration can be intimidating to the new user. There are a number of methods for easing the initial installation problems, but you are still left with an intimidating configuration process.
One option is to use Centreon, a (relatively) new front end for Nagios with a more accessible web front end for configuration. Centreon is fully open source and is supported by Merethis, a French company, who also sponsor development of the project.
Centreon is similar in concept to Groundwork Open Source though far narrower in scope.