U.S. History between the Civil War and World War I: Websites

Timeline of U.S. foreign relations from the Office of the Historian.

Collection documenting the U.S.'s changing territories from the University of Texas.

A search (at the top right of the page) on a given topic turns up plenty of information available online.

A wealth of online resources from the National Archives, including government agencies and specific events.

Another good collection of online resources for research, including specific sites and societies.

Statistics and fast facts sorted by decade on the U.S. Census' official site.