Hathi Trust Digital Library
The Hathi Trust organization has created a digital library for many public domain (and some copyrighted) materials. It can be searched by catalog, by collections, and by full text within a work; those items that have been digitized and placed on the site in PDF files will have a "Full View" check box underneath their search results. PDFs can be viewed freely and single pages of them may also be downloaded freely; downloading the entire work requires logging in as part of a partner institution.
Ebsco Ebooks Collection.