I just thought a bit about the restrictions the GPLv3 allows, and I think I just understood their purpose and effect for the first time (correct me, if I'm wrong :) ).
The GPLv3 allows developers (=copyright holders) to add selected restrictions, like forbidding the use of a certain brand name or similar.
The catch with them is, that any subsequent developer who adds anything is free to simply strip off the restrictions.
All content of these sites is under free licenses, except where explicitely noted otherwise.
This means you can use my works however you want (even commercially), as long as you allow and enable others (and me) the same with all the works you create from or using parts of my works, and say who created and modified the original works.
The works must stay under the same license(s).
To use them, you can (for example) just put this license text alongside them (i.e. as html page) and create a link pointing to it. Alternatively just put the following at a well-visible position on your creation:
Copyright [YEAR] [your name and other contributors] and [YEAR] Arne Babenhauserheide. Provided under free licenses, including GPLv2 or later, cc BY-SA, GNU FDL without invariant sections, Art Libre and the "Lizenz für Freie Inhalte". Details: http://draketo.de/licenses
More exactly: My works can be used (depending on the type of content) under the following free licenses: