Christian Classics Ethereal Library
Maintained by Calvin College, CCEL is a digital library of several hundred classic Christian writings in the public domain. All works are free to read online and many are available as free PDFs. When available, ePub downloads are $2.95 per title.
HathiTrust
A digital preservation repository featuring digitized content from several major U.S. libraries. Currently there are over 3 million volumes in the public domain that can be read online for free. To limit search results to public domain materials, check the "Full view only" box while performing a catalogue search.
Internet Archive: Ebook and Texts Archive
Over 3 million free ebooks and texts including many important historical and academic texts, all in a wide variety of download formats.
Oxford Text Archive
Nearly 3,000 primary source electronic texts and resources for literary and linguistic researchers.
Project Gutenberg
Over 46,000 free ebooks in a wide variety of download formats.
LibriVox has a dedicated group of volunteers who read and record public domain books, which are made available as audiofiles (MP3s) for free streaming and downloading.
An updated list of over one million freely readable ebooks, maintained by librarian John Mark Ockerbloom at the University of Pennsylvania.