GeekTool is an incredible tool that lets you put information on your wallpaper. It is not a tool for beginner users, since you must have some understanding of Unix and shell commands. Or you have to be very good at following tutorials and learning from them. What GeekTool basically does is let you put "Geeklets" on top of your wallpaper. There are three types of Geeklets: File, Image and Shell. The Shell mode lets you launch scripts or commands, Image mode lets you monitor bandwidth usage, CPU load, memory availability, etc. And the File mode lets you monitor log files or other types of files.
One great example of something you can make with GeekTool is found on the Mac App Store page for GeekTool. The screenshot in question shows a wallpaper with a sand dune. Below the sand dune, there is a clock that shows the time. At the bottom, you can see the computer's uptime, and at the top, some weather information. This is not easy to do if you don't know what you are doing, but there are tutorials that show exactly how it is done.
I have toyed with GeekTool in the past, and I have made some nice wallpapers with it. Even if you end up not creating anything great, GeekTool lets you create a wallpaper that fits your need. All you need is some research and creativity.