Barnes and Noble
Download classics like The Adventures of Serlock Holmes and Pride and Prejudice for free. The Kindle Store has a large library of out-of-copyright books. Subscribe to Amazon Prime ($99/year), and you'll gain access to thousands of new and recent-release titles. Membership also entitles you to a free book each month from both the Kindle Owners' Lending Library and the Kindle First program.
File Formats: Kindle. You can download a free app to read Kindle files on your Mac, PC, iPod, iPhone, Android or Windows Phone
Your Local Library
Prefer to read new-release books? Check to see if your library has an eBook collection that you can tap.
File Formats: Varies
This site was cataloging free eBooks long before eBooks were a big deal. All 45,000 titles are in the public domain.
File Formats: ePub, Kindle, HTML and text
Get all the copyright-free books that you'll find on other sites, plus new books that have been published under a Creative Commons agreement.
File Formats: ePub, Kindle, HTML and almost two dozen others