Geektool 3

GeekTool 3 is a System Preferences module for Mac OS X. It allows you to put widgets (geeklets) on your desktop.

At this time, 3 kind of Geeklets are supported : File, Shell and Image

File let you display the content, and evolution of a file on your desktop, very convenient for log files for example, you quickly see what is happening on your mac

Shell is certainly the most powerful plugin : Displays on your desktop the result of any shell script

Image let you show images coming from your local machine or internet, very usefull to display graphs from monitoring systems like cacti/mrtg/...