GeekTool available on the Mac App Store
GeekTool is a system preferences module that lets you display information like specialized calendars on your desktop via plugins. GeekTool also has a shell mode that lets you launch custom scripts or commands and an image mode to watch common system parameters such as CPU load and memory availability. The app is targeted towards advanced users with knowledge of unix and shell commands. If you don't want to write your own script, there's a variety of scripts (geeklets) available here.
GeekTool landed in the App Store a little over a week ago and is available here for free. If your desktop is boring and empty, check out GeekTool and put that unused screen real estate to good use.
