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.