From the home page of the Perry-Castañeda Library Map Collection you can access a wealth of maps of current and historical interest. Links in the left margin of the home page guide you to specific collections of the Perry-Castañeda Library Map Collection in addition to providing links to other sites for additional maps. The maps can be downloaded in PDF or GIF format.
The largest collection of cartographic, map, and geographic material in the world is at the Library of Congress Geography & Map Division. The Reading Room is available to everyone and includes digital collections.
Another great resource is the American Geographic Society Library which is located at the University of Wisconsin Milwaukee. Over 1.3 million items is in this premier collection of cartographic and geographic materials as well as some incredible preservation work.
US Geological Survey (USGS)
USGS Earth Explorer
A tools to "search, download, and order satellite images, aerial photographs, and cartographic products. In addition to data from the Landsat missions and a variety of other data providers, EE provides access to MODIS land data products from the NASA Terra and Aqua missions, and ASTER level-1B data products over the U.S. and Territories from the NASA ASTER mission. Registered users of EE have access to more features than guest users."
USGS offers a tutorial for this tool https://earthexplorer.usgs.gov/documents/helptutorial.pdf