Obviously this requires geektool.
You must install ImageMagick for this to work. I installed it through MacPorts, but a normal install may work.

This assumes everything in the archive is located at ~/.geektool. It will not work without it.
To get the folder to the right place and name:
copy or move the folder with this file in it so it's named gk and is in your home directory
open Terminal (Applications/Utilities) and enter: mv gk .geektool
The folder should disappear from Finder.

The only critical files are:
dot.sh - the script which makes the clock
0h.png - the hour hand at noon
0m.png - the minute hand at noon
f.png - the face of the clock

The three image files can be swapped out for others, though it'll take some tweaking for a good look.
The face in particular can be swapped for any of the other three clock faces - just rename f.png to something else and rename the clock face you want, like f2.png, to f.png.

Other files:
0bak.png - a simple clock hand - just a line, really
f0, f1, f2 - alternate clock faces
h.png - a file created on-the-fly by dot.sh, the position of the hour hand when the script was run
m.png - the same as above, but the minute hand
o.png - the actual clock image


The scripts you need in geektool
1 shell script:
/Users/username/.geektool/dot.sh (replacing username with your username, obviously)
This shouldn't have any output. 

1 picture, url:
file:///Users/username/.geektool/o.png
(You'll probably want to go into "style" and set it to "fit" and adjust the box so it's right.)

Now, an important note: the clock will only be updated once the script has run, and then the image has updated. This means that if you have both set to refresh every, say, 10 seconds, the clock can get up to 20 seconds behind. Not really a big deal.