Munin
Munin is used to graph server statistics such as memory usage, which is useful in order to understand overall server health. Since Munin graphs statistics over time, you can see resource allocation trends, which can help you find problems before they get serious. There are several other applications like Munin that create graphs; however, like Nagios, Munin is a good starting point.
It is designed to be easy to use and set up. The out-of-the-box experience gives you many graphs with little work.
In the legends of the Norse, Hugin and Munin were two pitch-black ravens. They tirelessly flew around the world of Midgard and collected information. After their journeys far and wide, they returned to the god king Odin to sit on his shoulders and tell him all about their experiences. The name Hugin derives from the word for thought and Munin from the word for memory.
While Nagios focuses on the high-level traits of the health of a service (whether the service or host is alive or not in binary...