The Cartographic Collection at the Boston Athenaeum is impressively displayed here. From the website: "This digital collection features representative items from the Athenæum cartographic holdings containing thousands of sheet maps, plans, and charts, and more than 600 bound atlases. They range in date from the fifteenth to the twenty first centuries in date. After more than 200 years of acquiring maps, the cartographic collection juxtaposes common maps with items of great rarity."
Note also their online finding aids.