USGS - U.S. Geological Survey:
   USGS: Maps, Imagery, and Publications
   USGS - Topographic Maps
National Geographic:
   Maps - National Geographic
    AllTrails.com
    Digital Maps
   Trails Illustrated Hiking and Recreation Maps - National Geographic Store
   Adventure World Maps | National Geographic
Discover Camping, Hiking, Mountain Biking & Other Outdoor Trails | AllTrails.com
Trails.com:
   Topo Maps & Aerial Photos | Free Topographic Map Data 
   Trail Maps | Find Trails for Hiking, Backpacking, Bikes, ATVs & More!
Maps for handheld GPS units
Most of the locations under places here have maps associated with them.
Map Apps for tablets and smartphones:
  iPhone Travel App, iPhone GPS Tracking | EveryTrail