Free downloadable historic maps of Chester County can be found at the Library of Congress, and at the New York Public Library among its state map collection (scroll down to find PA or a subcategory under PA). The Pennsylvania Historical and Museum Commission (PHMC) also has an online map collection. The Chester County Archives has an online map page that includes a 1777 property atlas and a section on maps at the Archives; the latter section also includes an online series of Chester County maps that shows the county's municipalities as they developed and changed from 1690 to 1880. The University of Texas Libraries have maps that are mostly freely downloadable in the Perry-Castañeda Library Map Collection, with many historic maps of the United States and links to maps on other websites.
Historical aerial photographs of Pennsylvania are available at the Historical Aerial Viewer. Zooming into a particular location can be done using a mouse wheel or by specifying a location, such as "Franklin Twp PA," and then selecting from the dropdown box. The map is populated with small clickable date icons ("40s") that link to a list of downloadable aerial photographs, which may be available in several formats. This site replaces the Penn Pilot website.
Historic Map Works has a wide range of maps for sale, but watermarked maps are also free to view. They have the 1873 Witmer atlas of Chester County and the 1883 Breou's atlas of the county. Or you can browse the entire collection. Another commercial source for images of PA and Chester County is Andy's Antique Atlases. The images are viewable as watermarked PDF files.
Other useful websites are those of Historical Maps of Pennsylvania and the Greater Philadelphia GeoHistory Network. The David Rumsey Map Collection is extensive and freely viewable, with a good collection of Pennsylvania maps.