Germany -Prussia- Poland
Useful links for German, Prussian & Polish research:
FamilySearch wiki for Germany genealogy - the place to start.
FamilySearch - search baptism, marriage and burial indexes and view their collection of digitised German records.
https://familysearch.org/search/collection/location/1927074?region=Germany
Evangelical Church Books Online. Search here to see if your parish register has been digitised and published online.
Also provides some basic information about parish registers and old German script and has links to other archives in Germany.
Look for the town/village your ancestor came from. This will also give you the name of the kreis (district) and province, and the polish equivalent of each if relevant.
Various other useful information.
Matricula - more online parish registers for Germany, Austria and Poland
High resolution old maps of Germany/Prussia
http://digicoll.library.wisc.edu/cgi-bin/German/German-idx?type=header&id=German.RavenAtlas
Search family trees and find other German genealogy information on this site.
International Tracing Service - Victims of Nazi persecution - documentation, research, information and commemoration
The Old Lutheran Emigration at the Middle of the 19th Century
Search for your ancestor
Online heritage books:
Excellent site for those with ancestors from the Poznan (Posen) region of the former Prussia.
Search marriage indexes
Schleswig-Holstein:
Dansk Centralbibliotek for Sydslesvig - The Schleswig Collection
Many resources for Schleswig history
Schleswig-Holstein:
Information on researching in Schleswig-Holstein including great information on the various archives in the region.
German - Australian Genealogy & History Alliance
Old German Type and Handwriting: