Euratlas Periodis is a fantastic web resource that depicts the history of Europe through maps, documenting the political situation at the end of each century from the first to the twentieth. It also functions as a historical gazetteer—when you zoom in, you can see sovereign states and dependent entities as well as political boundaries and important cities.
Membership to the website is free and allows users to download maps and pictures. To become a member, enter your email address on this page—Euratlas Periodis will then email you the log in information.