Google released an update to Google Earth recently (build 4.0.2416 (beta)), and it contains a bevy of historical maps:

-- World Globe 1790
-- North America 1733
-- United States 1833
-- Lewis and Clark 1814
-- New York 1836
-- Buenos Aires 1892

and several more. These maps are great for browsing and comparing current locations to older ones.

For example, I've been reading reviews of Steven Johnson's The Ghost Map recently. (Haven't read it yet, but I recommend Emergence.) The Ghost Map is about a cholera outbreak in 1854 London on Broad Street in the Soho district. Google Earth now offers a map of 1843 London, and the image below is taken from that.

And here's an image of how that area looks today:

A map of Paris, circa 1716:

San Francisco, circa 1853: