The King's Topographical Collection hosted on the Flick Commons contains more than 17,000 historical maps and images related to maps. The King's Topographical Collection is comprised of maps and drawings produced between 1500 and 1824. You can browse through, view, and download all of the maps and drawings in the collection. Unfortunately, the ability to search within the collection on Flickr is limited to just using "control+F" to search for words on the displayed page. When you do find something you like, click the download button on the image to save it in resolution of your choice.
Maps of Cities, hosted by the Library of Congress is one of two sets of historic maps available through the Free to Use and Reuse collections on the LOC's website. The other set of maps is called Discovery and Exploration. Both the Maps of Cities and the Discovery and Exploration collections contain about two dozen historic maps that you can download and reuse for free in any classroom project. All of the maps can be downloaded as JPEG files (three sizes available) and as GIFs.
topoView is a good place to find historical maps. topoView is a USGS website that provides historical maps dating back to 1880. You can download the maps in variety of file formats including JPG and KMZ. In the following video I demonstrate how to find and download historical maps on the topoView website.