GeekTool vs. NerdTool
Just my five cents to the discussion, if there is any: Last week I decided to give GeekTool a try. While searching for it, I saw NerdTool mentioned somewhere, but decided to go for GeekTool because it seemed more popular.
However, I now switched to NerdTool, mostly because it supports more colors:
The bright cyan would display as white in GeekTool. All in all, I think it works really well in between my Desktop Shelves :)
Another reason for NerdTool is that it takes up less RAM, though GeekTool doesn't need that much either, so that alone wouldn't have been a reason for me.
Reasons for GeekTool include that it's kind of nicer to set up the Geeklets because you can move and resize them right on the desktop. It also automatically uses the right language, with NerdTool I have to set it to German manually for each shell script, but that's not too much of a hassle as soon as you know how to.
There's a more elaborate comparison on http://www.leancrew.com/all-this/2011/09/nerdtool-over-geektool/; that site was also the reason I decided to try out NerdTool in the first place.