Munin the monitoring tool surveys all your computers and remembers what it saw. It presents all the information in graphs through a web interface. Its emphasis is on plug and play capabilities. After completing a installation a high number of monitoring plugins will be playing with no more effort.
Using Munin you can easily monitor the performance of your computers, networks, SANs, applications, weather measurements and whatever comes to mind. It makes it easy to determine "what's different today" when a performance problem crops up. It makes it easy to see how you're doing capacity-wise on any resources.
Munin uses the excellent RRDTool (written by Tobi Oetiker) and the framework is written in Perl, while plugins may be written in any language. 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). 
Simply put, Munin allows you to make graphs about system statistics. You can check out University of Oslo's Munin install to see some examples of what it can do.
Munin relies on a client-server model. The client is munin-node, and the server is munin (referred as to "munin-master" in the documentation).
You will only need to install munin-master on a single machine, but munin-node will need to be installed on all machines you wish to monitor. This article will focus on a single-machine installation. Further documentation may be found on the Munin documentation wiki.
munin and munin-node
There is currently a munin (munin-master) and a munin-node package in extra.
# pacman -S munin munin-node
Optional web server
The following guide sets up Munin to generate static HTML and graph images and write them in a directory of your choosing. You can view these generated files locally with any web browser. If you want to view the generated files from a remote machine, then you want to install and configure one of the following web servers:
Or one of the other servers found in the web server category.
Add the daemon to /etc/rc.conf:
DAEMONS=(syslog-ng network netfs crond httpd munin-node)
Run munin-node-configure with the --suggest option to have Munin suggest plugins that it thinks will work on your installation:
# munin-node-configure --suggest
If there is a suggestion for a plugin you want to use, follow that suggestion and run the command again. When you are satisfied with the plugins suggested by munin-node-configure, run it with the --shell option to have the plugins configured:
# munin-node-configure --shell | sh
Create a directory where the munin-master will write the generated HTML and graph images. The munin user must have write permission to this directory.
/srv/http/munin is used below, so the generated output can be viewed at http://localhost/munin/ provided that a web server is installed and running.
# mkdir /srv/http/munin # chown munin:munin /srv/http/munin
Uncomment the htmldir entry in /etc/munin/munin.conf and change it to the directory created in the previous step:
Before running munin, you might want to setup the hostname of your system. In /etc/munin/munin.conf, the default hostname is "myhostname". This can be altered to any preferred host name. The hostname will be used to group and name the .rrd files in /var/lib/munin and further, used to group the html files and graphs in your selected munin-master directory.
Run the following to have munin collect data and update the generated HTML and graph images every 5 minutes:
# crontab /etc/munin/munin-cron-entry -u munin
Munin should be able to run now. To make sure everything works, start munin-node:
Then run munin-cron manually to generate the HTML and graph images:
# su - munin --shell=/bin/bash $ munin-cron
And finally test the interface by pointing your browser to the output directory or http://localhost/munin/.
There are many Munin plugins out there just waiting to be installed. The MuninExchange is an excellent place to start looking, and if you can't find a plugin that does what you want it's easy to write your own. Have a look at HowToWritePlugins at the Munin documentation wiki to learn how.
Basically all plugins are added in the following manner (although there are exceptions, review each plugin!):
Download a plugin, then copy or move it to /usr/lib/munin/plugins
# cp <plugin> /usr/lib/munin/plugins/
Then link the plugin to /etc/munin/plugins:
# ln -s /usr/lib/munin/plugins/<plugin> /etc/munin/plugins/<plugin>
Now test your plugin. You do not need to use the full path to the plugin, munin-run should be able to figure it out:
# munin-run <plugin>
And restart munin-node:
# /etc/rc.d/munin-node restart
Finally, refresh the web page.
If you want to remove a plugin, simply delete the linked file in /etc/munin/plugins - there is no need to remove the plugin from /usr/lib/munin/plugins.
# rm /etc/munin/plugins/<plugin>