Metasites:  large sites that contain much information and often many other links

• Internet Modern History Sourcebook --  part of the Internet History Sourcebooks project by Paul Halsall at Fordham University.  Although it concentrates on important documents, it also provides many useful links.

• United States History from the WWW Virtual Library  --  find information by period or topic

• Links to the Past  --  an interesting collection of resources from the National Park Service, interesting section on getting grants and other financial assistance for preservation.

• United States Resources  -- from the Internet Public Library

• A Hypertext on American History  --  from USIA, good supplemental links.

• American History  --  from about.com

• The TimePage:  American History Sites  --  a comprehensive listing of sites developed by institutions and groups

Fast Facts:

• Infoplease.com  --  The Information Please Almanac online.  

• This Day in History  --  from the History Channel.  Find what happened today or on your birthday.  Also, many other interesting articles.

• Refdesk.com  --  a handy source of fast facts on all subjects, including history.

Chronologies & Timelines:

• DMOZ /Open Directory Project -- a variety of timelines, including U.S.

• U.S. History Timelines from Yahoo  --  on a variety of different topics

• U.S. History Outlines and Charts  --  an interesting group of charts and outlines, including timelines, on important times in U.S. History, from Greg Feldmeth.

• Timepage  --  indepth timelines of different periods of American History

Maps, Pictures, etc.:

• American Memory  --  a collection of many different types of materials, including pictures, maps, electronic documents, movies and audio, from the Library of Congress

• Library of Congress Online Exhibits --  lots of good information and graphics, current exhibits include Bob Hope, Religion and the Founding of the American Republic, and Blondie Gets Married.

• National Geographic Map Machine 

• U.S. Historical Maps from UT-Austin

• Utah Maps  --  includes one from 1805

• Historical Pictures Collection -- a wide selection from the University of San Diego History Department

Documents:

• Core Documents of U.S. History  --  from the U.S. Government

• AMDOCS --  Documents for the study of U.S. History, from the University of Kansas

• Douglass  --  Archive of speeches and related documents from Northwestern University

• National Archives and Records Administration (NARA)  --  a good place to begin if you're looking for government publications on any topic.

• University of Michigan Documents Center  --  an excellent place to look for statistics and links to most government publications.

Looking for something more specific?  Use a search engine like Google.