Is there a list or database or API for finding hot-spots and public available wifis for Germany?
Of course this data is volatile, so it should be often actualized.
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There is a wiki at http://freewifiwiki.net/index.php?title=Germany which is a listing (not a database). There are links at the bottom of that site to additional sites in German and English appear to augment this overarching listing.
In Germany there is an initiative called "Freifunk", which sets up free wifi networks everywhere. At the moment (August 2015) there are about 17K hotspots. There is a map showing all (public) hotspots and there are some APIs to get the data. The advantage of Freifunk is that it doesn't require a registration and it's uncensored.