From the Old Maps Online Web Site:
Use your iPhone, iPad or any Android device to view over 250,000 historical maps from the 15th to the 20th century.
The Old Maps mobile app can display the relevant maps according to the places of interest to you. Don’t like travelling yourself? No worries, you can still use it from home. Search for your place of interest by name, or simply zoom and pan, to get the most accurate results instantly.
The app and access to the material is free.
Hat Tip: Europeana eNews
See Also: List of Participating Organizations