Yes, you can use it for free and you can freely download the source code if you want to customize it. However please remember if you make interesting adaptations that may be useful to others, please send me your adaptation.

Those graphs are not generated by GeekTool, but by products mostly based on the RRDTool graphing library: Cacti, MRTG or RRD.

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