The macosxhints Rating:
[Score: 8 out of 10]
GeekTool's interface is a bit confusing and can be somewhat daunting to understand, but it gets easier the more you use it. You create new entries in groups, and then assign a task (display a file, output from a shell command, or show an image) to that entry. Each of the types of GeekTool tasks has its own settings, along with universal options for update interval, window size, position, color, and (if applicable) text. You can even specify any window as an "always on top" window, in which case it will float over all other OS X objects (including the dock).
Entering the update intervals is more difficult than it need be. The only time interval allowed is seconds, which makes for lots of math when you want to update something twice a day, once a week, once a month, etc. A better solution would be a numeric entry box and a pop-up menu for choosing between seconds, minutes, hours, etc.
Once a task has been created, you can move and size its window by either entering the data in the Location portion of the input screen, or by dragging and stretching its window with the mouse -- just make sure you select the task in the Groups window first.
Going into a ton of detail on everything you could do with GeekTool will take more time than I have available. Probably the best thing to do is to download it and start playing with it. Start with a live update image or the display of a file's contents, as those are probably the easiest two tasks to create. If you're comfortable with those tasks, try a simple shell command -- uptime or ls -l, just to see how it works. Note that GeekTool displays the output of a command after it has been run and terminated; hence, it won't display the output from top, since that's an interactive process. But you can have it display the output from top -l 1, since that just runs once then exits.
Given the shell's ability to run AppleScripts, the Shell tasks are by far the most powerful options in GeekTool. You can see an example of that in the iTunes hint linked above ... and if you're a GeekTool power user and have a favorite shell task, feel free to post it here in the comments!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040706234745708