Ganglia is a graphing and monitoring solution for large clusters. It can aggregate information in convenient overview displays.
The word "ganglia" is the plural form of ganglion, which is a nerve cell cluster in anatomy. The analogy implies that Ganglia can be the sensory nerve cell network in your cluster.
Like Munin, Ganglia also uses RRDs for database storage and graphing, so the graphs will look similar to the previous Munin graphs. Code reuse is a good thing!
Ganglia has an interesting online demonstration at http://ganglia.wikimedia.org/latest/.
Wikimedia serves media content for Wikipedia, which is pretty busy. The demonstration thus gives us a good overview of Ganglia's capabilities, which would be hard to get in any easy way in your ...