Munin is a networked server monitoring tool written in Perl. Munin consists of two parts: the Munin server and Munin nodes. The nodes are running on the servers we want to monitor, while the server periodically (5 minutes by default) fetches the data from all of the registered nodes. The server is a able to build charts from this node data, which gives insight in the cpu usage, disk usage, network load and more.
The Munin team has a live demo of Munin, which can be viewed at http://munin.ping.uio.no/ I have installed both the Munin node and server on my Ubuntu server machine, and added nodes of other servers as well.