GeekTool Available in Mac App Store

GeekTool has been around on Mac OS for some time now, but now, it's available on the Mac App Store. It allows you to customize your desktop with Geeklets. These little Geeklets sit on your desktop. They're unobtrusive and sit in the background. There are three types of Geeklet: File view, Image, and Shell Script. File Geeklets allow you to monitor the contents of a file. It's great if you like to keep an eye on your logs.
The image is pretty straightforward. It shows an image on your desktop. It might not seem immediately useful, but it can be used for clocks, calendars, or, in the screenshot below, a layered desktop.

Finally, there's the big one: shell scripts. It's what allows you to create calendars, weather data, clocks, and more. If you can't create a script yourself, fear not, there are plenty you can download from here. If you have some programming knowledge, it's really not too hard to start modifying downloaded Geeklets, and eventually creating your own.

There's a small learning curve, but with just a little work, you can customize your desktop quite easily. My desktop was created with a nice wallpaper (the 2012 Focus ST), Nocturne to create a black menubar, Dock Library and Leopard Docks to use a dark dock, and two Geeklets that I modified slightly. The originals can be found on the site I linked to above. You can download GeekTool in the Mac App Store here.