"Millions of Documents at Your FIngertips"! Maps from the 1600 to the 20th century or freely available for viewing and downloading courtesy of the National Library of France. Other resources available at the site include books, manuscripts, images, periodicals, sound recordings, and scores. Download maps in PDF or JPG. English language access is not consistently available across the site, but appearance and web site conventions are often an adequate guide for successful use.
Also, there's an app for that! (Available at the site.)
What's new in maps at Gillica?