Many manually-transcribed German directories can be searched at http://www.adressbuecher.net/.
For the complete list of places included, see http://www.adressbuecher.net/place/list. There is some coverage outside the borders of today's Germany (e.g., Poland, South America).
Volunteers interested in helping the ongoing transcription project should visit http://wiki-de.genealogy.net/Index.php/ ... %C3%BCcher.
-Logan