Maier, Nicolas. International Exhibitions and World Fairs [medals]
ExpoLinks (Marc Wiliams)
Coney Island and the Expositions (Malachy McCauley)
National Gallery of Art, Photographic Archives: Photographs of International Expositions
Online Archive of California: [type in "expositions" in search box to get a general set of more than 2000 images from different expositions. Can narrow search later]
Wikipedia,The Free Encyclopedia: List of world's fairs.
archInform: World Exhibit
Coenen, Stephen T. (StevieTV.com). World Fairs of the 1900's

1893 World's Columbian Exposition
The World’s Columbian Exposition:  Idea, Experience, Aftermath
Julie K. Rose,  hypertext, interactive M.A. thesis, University of Virginia
1893 World's Columbian Exposition (Chicago)
Gair, Christopher: Whose America?:  White City and the Shaping of National Identity, 1883-1905 [click on Essays, then White City]
Hancock, Scott.  Web-Book World's Columbian Exposition Resources Links Page
Schulman, Bruce R.  Interactive Guide to the World's Columbian Exposition
Illinois Institute of Technology, Paul V. Galvin Library Digital History Collection: World's Columbian Exposition of 1893
Internet 1996 World Exposition: Voices from the 1893 World's Columbian Exposition
Rose, Julie K. City Beautiful: The 1901 Plan for Washington D.C.
 

1933-1934 Century of Progress Exposition
Brooks, Monica G.:  The Good Housekeeping Stran-Steel House Chicago Worlds' Fair, 1933
Duncan, Ray.  Neonatology on the Web: Century of Progress Exposition
PBS:  American Experience | Streamliners | People & Events
People & Events: Chicago Century of Progress Exposition (World's Fair), 1933-1934
Mayton, Don M. (project director). National Automotive and Truck Museum of the United States. 1933-1934 Chicago World’s Fair:  GM Futurliner Restoration Project
Fae, Lynne (?): A Century of Progress International Exposition Chicago 1933-34.
Internet 1996 World Exposition: Voices from the 1933 Chicago World's Fair
Kidd, Dustin: Dissemination of Order in Chicago's Century of Progress

Fae, Lynne?: Chicago World’s Fair: A Century of Progress Exposition 1933-1934

1939-1940
1939 World's Fair (New York)
Anderson, Alan.  New York World's Fair 1939-1940
Rose, Julie K.  The Iconography of Hope:  The 1939-40 New York World's Fair
New Deal Network.  New York World's Fair, 1939
Voices from the 1939 New York World's Fair
The New York Times: Looking Forward, Looking Back: 1939, Building the World of Tomorrow At the New York World's Fair.
1939 and 1940 World's Fairs

Van Dort, Paul M. 1939 New York World's Fair

Free Library of Philadelphia: Centennial Exhbition Digital Collection

Omaha

Trans-Mississippi Exposition Historical Association, Inc.:  The 1898 Trans-Mississippi Int’l Exposition and Indian Congress and the 1899 Greater American Exposition

The 1894 San Francisco Midwinter Fair
Bancroft Library, University of California, Berkeley: The Way California Could Be: the Panama Pacific International Exposition
Panama Pacific International Exposition 3D

Rau, Scott: The Panama Pacific International Exposition

The Paris Exhibitions – Exposition Universelle de Paris 
West, Michael J. Spectacular Ideology: The Parisian Expositions Universelles and the Formation of National Cultural Identity, 1855-1937 

1900
African American Photos for Paris Expositions 1900 Library of Congress, Prints and Photographs Division
National Gallery of Art.  Anatomy of an Exhibition:  Art Nouveau, 1890-1914.  The Paris World’s Fair, 1900

Provenzo, Eugene F.: Paris 1900: The Exhibit of American Negroes
1925
Chandler, Arthur. Virtual Visit to the 1925 Paris Exposition Des Arts Décoratifs
1931
Chandler, Arthur. Empire of the Republic: The Exposition Coloniale Internationale de Paris, 1931