Library of Congress Digital Collections
New York Public Library Digital Collections--select 'Digital Collections'
Heidelberg University Library Digital Library
British Library--under 'Collections' select 'Digital Collections'
Gallica, Bibliothèque nationale de France
Princeton University Digital Library
[Harvard Library Digital Collections] (http://library.harvard.edu/digital-collections)
Kind of vague question about a very big category. I presume you know there are loads of out-of-print books and texts at Google books, the Open Library and Project Gutenberg. Gutenberg is pretty good, but the unedited OCR process at the Open library has resulted in some scrambled pages being posted. Google has also scrambled some of its online books.
Legacy tobacco documents MASSIVE database, makes the tobacco companies look like what they were capitalist Nazis. Also, detailed enough for researchers but easy enough to search for the common man.