GeekTool is a PrefPane (System Preferences module) for Panther or Tiger to show system logs, unix commands output, or images (i.e. from the internet) on your desktop (or even in front of all windows).

Use File mode to monitor MacOS X activity with /var/tmp/console.log or /var/log/system.log, or Shell mode to launch custom scripts or commands like "df" to check space left on filesystems, "uptime" to monitor load of your machine... FInally, Image mode helps you monitor bandwith usage, CPU loads, memory availability of your server, via tools like MRTG or RRD.

MenuMeters is a set of CPU, memory, disk, and network monitoring tools for Mac OS X. Although there are numerous other programs which do the same thing, none had quite the feature set I was looking for. Most were windows that sat in a corner or on the desktop, which are inevitably obscured by document windows on a PowerBook's small screen. Those monitors which used the menubar mostly used the NSStatusItem API, which has the annoying tendency to totally reorder my menubar on every login.