Munin (http://munin.projects.linpro.no/) is a great tool, but there are some conceptional problems with it:

- you dont want to have another "monitoring service" listening on the machines you want to monitor

- relatively complicated to setup monitoring on a host and then adding to a central monitoring server. Can be much easier.

- no security for transmitted monitoring data

those problems should be addressed in this project:

- creating of very slim monitoring client that is run via cronjob

- creation of a data sink RRD server that collects all incoming data and graphs it.

- no setup required on the server to add another host to monitor - it will figure it out itself

- XML-RPC interface to submit monitoring data in order to support any client you might want to write.

- slim software: code is currently under 200 lines and fully functional

- its SIMPLE!

have a look at the quickstart guide