Gnaglios is a project to enable easily adding nagios alerts based on ganglia data in large environments. It's overkill for small shops but when the number of nodes and metrics in ganglia gets large, it provides an efficient method for keeping your nagios checks fast while using current data from ganglia.
More detail on a comparison of the various methods of integrating ganglia and nagios is available in the github.com/ganglia/monitor-core wiki: Integrating Ganglia with Nagios
The rest of the README describing how to use this project is in the debian/README file.