Maps and Atlases are useful for seeing a town and its context: the neighboring towns and distances to them, as well as the larger geographical demarcations of the time. Gazetteers similarly provide information about a city, its neighbors and jurisdictions. Some maps will be more recent and require you to trace a location using older maps or gazetteers in order to find historical place names.
General Online Maps
Any one of these could have collections of pertinent maps not listed in the areas below.
FEEFHS has a great collection of German maps from different time periods.
Euratlas has maps of Europe beginning in 1 A.D. This can help you determine which in country your research should begin, based on borders in the time period of your research.
Generalkarte von Mitteleuropa are very detailed maps and have maps from parts of Germany, Poland, Russia, Czechoslovakia, Austria, Hungary, Romania, Ukraine, Italy, Yugoslavia Bulgaria, Albania, Greece and Turkey. To determine correct longitude from Greenwich, subtract 17º40' from the longitude coordinates shown on these maps.
Google Maps is a quick and easy way to see distance and location of a city.
Before 1871, Germany consisted of independent kingdoms, principalities, duchies and states. The borders of all have changed considerably over time. In 1871, all of the German states were consolidated, with the exception of the Austrian Empire, into the German Empire. Because of these changes, you might not be able to confirm a town name in old documents with modern maps.
Maps
This Wikipedia page has detailed maps of historical German provinces as well as links to the individual provinces, duchies or kingdoms.
In European German speaking countries, most families remained within a ten mile radius (~16 kl) of their hometown. A detailed map of that area could be very useful in identifying the Kleinstadt (small towns) that make up a parish. Maps can supplement gazetteers and geographical dictionaries by showing the physical distance between known ancestral towns and newly discovered towns. They also show relationships between the larger towns often mentioned and used for civil registration and the smaller towns associated with your ancestor. For example, a person can be born in a smaller town, but the parents will register that record in the larger civil registration office. Both names of towns will be on the birth certificate, but your ancestor was born in the smaller town. Many times towns with common names can be differentiated by referencing the larger registration offices.
Atlases
Meyers Gazetteer has a large collections of maps associated with the website. Once you have found the entry for the place you are interested in, click on the map to see your place in relation to nearby cities.
Place Name Indexes: when reading a historical German document, using a alphabetical index to locate the village might be necessary. Dr. Roger Minert has both forward and reverse indexes to help when only a partial name of the village is legible. They are also helpful for German-American research when the priest or scribe is guessing at the spelling of your ancestors home town.
Ravenstein Atlas des Deutschen Reichs by Ludwig Ravenstein is a collection of maps of Germany in 1883 and includes German speaking areas in Western and Eastern Europe. An important part of this Atlas is that small towns and villages are included and can be located. It also has locations of churches and statistics on religious denominations.
SwissTopo.com gets it's data from the topographic map of Switzerland from 1856. It is a comprehensive collection of all first editions and revised versions of the official Dufour Map, the Siegfried Map and the series of national maps.
Germans From Russia Settlement Locations: This website gives the modern day placement of German settlements in the Imperial Russian and Austria-Hungary empires that began in the 1700s and continued into the early 20th century just prior to WWII. Theses maps include present-day countries of Azerbaijan, Bulgaria, Georgia, Hungary, Kazakhstan, Kyrgyzstan, Moldova, Poland, Romania, Russia, Serbia, Tajikistan, Turkmenistan, Ukraine and Uzbekistan.
Like Germany, Poland also has a long history of border change, including being partitioned out to surrounding countries between 1795-1918, only to receive their statehood again after WWI. Additionally, Poland was Poland-Lithuania between 1648-1764.
Due to the Partitions of Poland between 1795-1918, depending on your area, you will need to search in maps of Russia, Prussia and Austria. Topographic Maps of Eastern Europe has great map collection from this time to help narrow down your ancestral area.
Ukraine
Old Maps Online particularly has a good collection of historical Ukrainian maps.
Alabama Maps has a good collection of Baltic maps.
Belarus
Belarus can include lands that were part of Lithuania, Poland, The Polish-Lithuanian Commonwealth, Russian Empire, Russia and the Ukraine and had large settlements of German speakers.