Geogen, which stands for Geographic Genealogy, is a project created and maintained by Christoph Stoeppel. Based on telephone book entries he created a mapping tool for family names in Germany (in its present-day borders). While it might not be useful for very common family names, such as Schmidt, Müller, or Schulze, it can be helpful with rather seldom family names. If one of those family names is especially heavy concentrated in one county, the immigrant ancestors might origin in this county.
There are two versions available, the simpler version 3 and the more advanced version 4 (with the latest bells and whistles).