[bradleyland]

Interestingly, an out-of-the-box Munin configuration on Debian contains nearly all of these. I recommend setting up Munin and having a look at what it monitors by default, even if you don't intend to use it as your monitoring solution.

[hansjorg]

Installation on Debian/Ubuntu is also as simple as installing the munin package (munin-node for subsequent hosts) and pointing a webserver at the right directory.

Extremely valuable when something is acting up.