This is just an incomplete list of some of the many online sources for digital texts and images (along with some meta-sources indexing and providing search portals for them). They're in no particular order.
See also Thoughts on Google Books.
But download patent images via http://www.pat2pdf.org/ for original USPTO images without further Google image processing. They of poor enough quality as-is. (In an act of cultural vandalism, the USPTO scanned the originals at 300dpi bi-level and then sent the originals to the landfill.)
A consortium of libraries. Most of their works are from Google Books digitizations from member libraries, but a few are from other (higher quality) digitization projects. Downloads of public domain materials are limited to individual pages if you're outside of a participating institution.
That their collection is extraordinary goes without saying. That they are now licensing their own images (if copyright by the Trustees of the British Museum) under the Creative Commons Attribution-Noncommercial-ShareAlike 4.0 license is commendable.
Note also that for content beyond this early material, which is available normally only via subscribing institutions, JSTOR now has a limited user access program that, for occasional research, can be quite useful. See http://about.jstor.org/individuals At the time of writing (2015), this access allows you to view three articles at a time on a virtual "bookshelf"; you can take articles off the "bookshelf" after 14 days. They calculate that this limits you to 78 free articles per year.
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