There is a very good overview on the following page:
http://prepaid-data-sim-card.wikia.c...European_Union
The page mentions Lycamobile Germany as one of the options. As far as I could see, Lycamobile offers more or less the same roaming offer since few months in all of the countries they cover, so it does not have to be Lycamobile Germany if one prefers a number of a different country.