With the opening of the Amazon.de Kindle store has come the opportunity for free books. Now my German is a little rusty so hopefully I've got this right (feel free to post corrections and I will update this post).
You can browse a complete list of Kindle free books here:
http://www.amazon.de/gp/search?node=...iscounterde-21
The top 100 free bestsellers (German and English) can be found here:
http://www.amazon.de/gp/bestsellers/...iscounterde-21
or you can subscribe to an RSS feed here:
http://www.amazon.de/gp/rss/top-free...iscounterde-21
Mostly public domain at the moment - although a few english language time limited freebies (which are free elsewhere have crept in).
Just want the English language free Kindle books? Browse from here:
http://www.amazon.de/gp/search?node=...iscounterde-21
There is also an english language only bestselling free books list:
http://www.amazon.de/gp/bestsellers/...iscounterde-21
and the associated RSS feed:
http://www.amazon.de/gp/rss/top-free...iscounterde-21