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GeekTool 3.1.1
Is GeekTool really free?
Yes, you can use it for free but you can no longer download the source code.
Where do those great graphs come from in the screenshots?
Those graphs are not generated by GeekTool, but by products mostly based on the RRDTool graphing library: Cacti, MRTG or RRD.
How can I put a show below text?
The shadow setting is available in the standard font & size Mac OS X panel, in the top tool bar
How can I run "top" in GeekTool?
Since top loops on itself by default, GeekTool will not show any output until the execution ends. But you can use top's argument "-l" to make it run a limited number of samples, like : "top -l2".
Thanks to Toni (http://toni.uebernickel.info/) here is a method to get top information in GeekTool :

top -n43 -l2 -o-CPU > top.txt && tail -n52 top.txt

What does it make?
It uses top printing 43 processes sorted by CPU usage descending and takes 2 samples.
Afterwards it prints the results of top in top.txt overwriting this file each time, so nearly no overhead is generated.
Now I want to have the last sample printed with GeekTool, therefor I use tail with the created file.
To see the statistics of top (count of running processes ...) you need to display exactly 9 lines more as the process list (amount of processes listed by top).

Why 2 samples?
Using only one sample displays no CPU Usage, because this is calculated as a delta value between certain samples.

The requirement is pretty easy:
the -n option of tail needs to be 9 more than the -n option of top (9 line of the print above process list)
A little bit of hard disk for the textfile :P
I see bunch of applescript and Adobe Script related errors in the console
On 64 bit OS with Adobe products installed, you see that kind of errors in the console :
2010-10-09 10:08:12.472 osascript[925:903] Error loading /Library/ScriptingAdditions
/Adobe Unit Types.osax/Contents/MacOS/Adobe Unit Types:  dlopen(/Library/ScriptingAdditions/
Adobe Unit Types.osax/Contents/MacOS/Adobe Unit Types, 262): no suitable image found.  Did find:
This is due to Adobe releasing only 32 bit version of their script environment. The good news is that a 64 bit version is available on their web site : http://kb2.adobe.com/cps/516/cpsid_51615.html