The most current 7.5' topographic quadrangles downloadable in .pdf format. If you're on your own computer download the Acrobat Reader plug-in that's offered. It will give you some useful geogaphic tools that will then be available for any georeferenced pdf.
Interactive map for the entire country with over a hundred layers of information.The National Map is a collaborative effort among the USGS and other Federal, State, and local partners to improve and deliver topographic information for the Nation. Presents a seamless topographic map of the US at 1:24,000.
The historical 15' quadrangles in .jpg format. Many are from the late 19th century. Only about twenty Eastern states are covered at the Maptech site. For other states check Odden's Bookmarks or the Perry Castaneda site.