Open Library

The site has indexed more than 20 million titles, with the goal of having a listing for every book ever published.   The listings include 1.7 million books in the public domain.  You can download many of these in a variety of formats – PDF, ePub, MOBI, DAISY, DjVu and ASCII text.  Others can be “borrowed” by providing an email address for checkout.  Note that some titles are still copyright protected and have links to sites for purchase.