I just came across a tremendous collection of historical German photography, bpk. Its images can be downloaded for a fee, but you can look at samples for free (those come with a watermark, though). Here’s are some noteworthy sets by various photographers.
Arthur Grimm, Hanns Hubmann, Gerhard Kiesling (wonderful colour photographs from East Germany), Willi Moegle (for those who like product shots), Erich Salomon, F. Albert Schwartz. The full list of photographers can be found here. There also exists a portal of images sorted by subject matter.