Digital Archives

A list of free databases (some may require a free account or library card).

Europeana, over 53,500,000 artworks, artifacts, books, videos and sounds from across Europe
Archive.org
HathiTrust
US National Archives
Library of Congress Digital Collections and as American Memory
National Park Service
New York Public Library's digital collections include audio, video, color maps, and many photographs
The European Library massive archive of materials from dozens of European libraries. Check out The Ryhiner Map Collection to find over 16,000 maps of the world from the 16th to the 18th centuries

From Universities

Story Corps
Mukurtu (MOOK-oo-too) is a grassroots project aiming to empower communities to manage, share, preserve, and exchange their digital heritage in culturally relevant and ethically-minded ways

Bracero History Archive collects and makes available the oral histories and artifacts pertaining to the Bracero program, a guest worker initiative that spanned the years 1942-1964
History Pin, an interesting user-generated archive of old photographs pinned to their geographical locations.

Here is Tulsa

US Fish and Wildlife Digital Library
Oklahoma's Digital Prairie, a collection of archives about Oklahoma history. Collections include photographs, and the Tulsa Race Riot
American Journeys contains more than 18,000 pages of eyewitness accounts of North American exploration, from the sagas of Vikings in Canada in AD1000 to the diaries of mountain men in the Rockies 800 years later
Digital Archive of American Architecture
Early Americas Digital Archive, a collection of electronic texts originally written in or about the Americas from 1492 to approximately 1820
Oyez, audio from the Supreme Court proceedings, oral arguments

Lots more here