munin

http://munin.projects.linpro.no/

It displays a nice set of graphs on a lot of system processes, that I really never thought about graphing before. Similar to "Quicklook"

http://bliki.rimuhosting.com/space/knowledgebase/linux/miscapplications/Quicklook

To install on a Debian based system just run:

apt-get install munin munin-node

It will be installed in /var/www/munin/ by default, so make sure Apache can pick it up there.

If your installing it on your new Ubuntu Hardy system, be aware of this bug, which will probably be resolved by the time you read this:

https://bugs.launchpad.net/ubuntu/+source/munin/+bug/41043