Aggregators by Subject

General

Endangered Archives Programme (British Library) - Links to a large number of online collections from archives around the world

World Digital Library - The Library of Congress’s online digital database of digitized books from around the world

Digital Archives Guide

Institute of Historical Research (School of Advanced Study, Univ. of London)

Freely available books

Databases for e-books and open access journal access

Internet Archive Book Images on Flickr - Over 2.6 million searchable images taken from books in the public domain

Radical online collections and archives (New Historical Express) - communism, socialism, labour, anti-racism, etc.

HathiTrust Digital Library - Millions of books online. Especially good on the Americas

Internet archive -  huge collection of books, films, websites, etc.

JSTOR Community Collections

JSTOR Primary Sources

Medical Databases

World Health Organization - 1950-present, reports and publications

Contagion: Historical Views of Diseases and Epidemics - Digital collection from Harvard University Libraries

Medical Humanities and History of Medicine (McMaster University)

Medical Heritage Library - Images, books, etc. - a fantastic resource

Medicine in the Americas - Digital library of texts covering evolution of American medicine, 17th-20th centuries

Wellcome Library, Digital Collections - topics include asylums, food, sex and sexual health, genetics, public health and war

amfAR Archives - AIDS archives

Resources History of Western Medicine in China, Resources Portal (IUPUI)

Culture

Getty Images - Online archive of still and moving images (fee for use in publications)

Visualizing Cultures (MIT) - resource for image-driven scholarship and learning

By Era

Pre-1945

Early Modern Resources - A research portal for the period 1500-1800

Harry Ransom Center Digital Collections (University of Texas) - c. 15th-17th centuries

Primary source databases relating to World War II, compiled by Daniel Todman (QMUL)

Post-1945

Cold War International History Project (Woodrow Wilson Center)

Vera and Donald Blinken Open Society Archive

Hoover Institution Digital Archive

History Lab - Freedom of Information Archive

WikiLeaks - A huge, largely unsorted and unread collection of sources for contemporary histories