Google Books - books with copyright restrictions are reproduced as 'snippets', or excerpts. Books in the public domain are usually reproduced in their entirety.
Hathi Trust - consists of digitized collections contributed by member libraries, hosted by the University of Michigan.
Internet Archive - digitized books, government documents, audio and video, web sites, and more.
Project Gutenberg - over 45,000 free, public domain, ebooks.
American Memory Project (Library of Congress) - maps, photgraphs, newspapers, govenrment documents, diairies and correspondence of immigrants, travellers, soldiers, etc. and more.
Digital Public Library of America - online collections of books, images, maps and more.
The Making of America - courtesy of Cornell University. A free, online, fulltext collection of nineteenth century American monographs, magazines and periodicals.
University of Wisconsin Digital Collections - primary resources for U.S. and Wisconsin History.
Denver Public Library Digital Collections - Denver and Colorado History in photographs, maps and documents.
New York Public Library Digital Collections - images, prints, videos, maps, and documents on a wide variety of historical topics.
The National Archives -
U.S. Government Printing Office