Munin is written in Perl and uses the RRDtool to create graphs, which are accessible over a web interface. Its emphasis is on plug and play capabilities. About 500 monitoring plugins are currently available. Using Munin you can monitor the performance of your computers, networks, SANs, and applications. It is intended to make it easy to determine "what's different today" when a performance problem crops up and to provide visibility into capacity and utilization of resources.
Munin has a master/node architecture in which the master connects to all the nodes at regular intervals and asks them for data. It then stores the data in RRD files, and (if needed) updates the graphs. One of the main goals has been ease of creating new plugins (graphs).