One useful resource for genealogists and other researchers are historical maps such as atlases. The Westchester County Archives has an extensive maps and plans collection, including several county-wide atlases. This collection includes an 1867 Atlas published by F.W. Beers which covers all of Westchester county, plus parts of New York City.
For a current map of Westchester County, consult the Westchester County Geographic Information Systems' Web site.
Note: the images linked to here are from a copy of the atlas held by the Westchester County Historical Society used for our first Virtual Archives collaboration, about the Civil War in Westchester County, rather than the Archives' copy of the atlas.