National Geographic, in collaboration with the USGS, has released a free to use web interface for locating and printing topographic maps of the United States.
 
Here are some of the features of this new service:
 
Index map page showing the quad maps in context
 
Standard USGS quad maps pre-formatted for letter size printing
 
Hillshading added for easier topographic visualizing
 
Visit the Nat-Geo website here: http://www.natgeomaps.com/trail-maps/pdf-quads