Munin is a system monitoring tool which allows you to monitor your
server or whatever remote via a webinterface. It is also able to monitor multiple servers.
To create graphs it uses
RRDtool.
The default installation on a debian system comes with a bunch of plugins for acpi, postfix, atp, processor, uptime and many others
which can be easily used together with munin. The best thing is that the plugins are only shell scripts combined with standard
unix tools like awk, sed and some perl which makes it easy to write your own plugins to monitor whatever you want via the webinterface.
For example the uptime plugin does not more than to execute
awk '{printf "uptime.value %.2f\n",$1/86400}' /proc/uptime and gives the output to munin.
You can see a list of example servers which use munin on
http://www.linpro.no/projects/munin/example/.
This one is nice.
I tested it for a while now on my fileserver and installed in on other systems as well and it seems to be great and easy to use.