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GeekTool is a PrefPane (System Preferences module) for Mac OS 10.5. It let you display on your desktop different kind of informations, provided 3 default plugins :

File plugin to monitor MacOS X activity with /var/log/system.log, or any file that you want to follow.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.

If you are looking for useful scripts, there is a great repository here :
http://www.macosxtips.co.uk/geeklets/