- Open Library
A digital library catalog with records for 23+ million books and full-text for over 1 million. Sponsored by the Internet Archive.
- Internet Archive
A large digital collection which includes 1.6+ million digitized books/texts.
- Digital Book Index
This site provides access to electronic books from many of the most important web e-book collections. The great value in this smaller selective site is that it includes texts from a wide range of digitization projects. Access is free but very brief registration is required.
- HathiTrust Digital Library
HathiTrust is a very large scale collaborative repository of digital content from research libraries including content digitized via the Google Books project and Internet Archive digitization initiatives, as well as content digitized locally by libraries.
- Humanities Text Initiative from the University of Michigan
- International Children's Digital Library
- Librivox (audiobooks)
- National Academies Press
Browse and search approx. 4000 online editions of NAP titles on a wide range of topics including education, engineering, energy, health, industry, math, nutrition, security, and more. Note: Not all titles are available free and there is no way to limit your search to only free titles.
- Project Gutenberg
A collection of some 36,000+ electronic books in the public domain.