Books Should Be Free
provides audio recordings of hundreds of books in the public domain that are available for online listening or downloading to your computer and or iPod.
Flat World Books
provides study tools, like flash cards and practice quizzes.
Flexbooks
offers extensive titles in math and science that are customizeable, easy to update, and free.
Follett eBooks
Gale Virtual Library of eBooks
MIT
has free online courseware available in math, science, and the humanities.
Planet eBook
has classic literature available as a free download.
Project Gutenberg
free electronic books.