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GeekTool is a PrefPane (System Preferences module) for Mac OS 10.6. It lets you display various kinds of information on your desktop via 3 default plug-ins:

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 or "uptime" to monitor load on your machine. Finally, image mode helps you monitor bandwith usage, CPU load, memory availability on your server with tools like MRTG or RRD.

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