University of Texas Map Collection.
This is the best one.
National Geographic's MapMachine
Good for battles among other historical maps. Use the navigation bar on the left and click on "historical map."
Map Collections: 1500-2002.
From the Library of Congress, this is good for cultural, transportation, battles, and cities and towns.
Identifies several key collections of historical maps.
This collection is from Yale University. It is arranged by named collections.
Good for reproductions.
National Atlas of the United States.
Interactive on a variety of topics such as ethnicity, crimes, agriculture. Arranged by continent and then by region. Of special note are the economic and physical summary pages. For example, the economic summary lists the GNP, inflation rate and daily calories per capita. Also, maps on population distribution, temperature and precipitation.