Alex Catalogue of Electronic Texts - This is a collection of public domain and open access documents with a focus on American and English literature as well as Western philosophy. Its purpose is to help facilitate a person's liberal arts education.
Baen Free Library - Baen Books is now making available - for free - a number of its titles in electronic format. Anyone who wishes can read these titles online - no conditions, no strings attached.
Calibre - Ebook Management software - Calibre is an e-book library manager. It can view, convert and catalog e-books in most of the major e-book formats. It can also talk to a few e-book reader devices. It can go out to the internet and fetch metadata for your books. It can download newspapers and convert them into e-books for convenient reading. It is cross platform, running on Linux, Windows and OS X.
Cornell University Libraries Public Domain Texts - These books, the originals of which are in the Cornell University Library, were digitized in 2008 with funding by Microsoft Corporation. Scanning was performed by Kirtas Technologies; OCR was performed by, and derivative formats created by, the Internet Archive.
Google Books - Search for a keyword in Google Books then click 'Public Domain only' to download lots of free ebooks.
Internet Archive Open-Access Text Archive - This collection is open to the community for the contribution of any type of text, many licensed using Creative Commons licenses.
Library of Congress Historic Books Online - Nearly 60,000 books prized by historians, writers and genealogists, many too old and fragile to be safely handled, have been digitally scanned as part of the first-ever mass book-digitization project of the U.S. Library of Congress (LOC), the world's largest library. Anyone who wants to learn about the early history of the United States, or track the history of their own families, can read and download these books for free.
LibriVox - LibriVox provides free audiobooks from the public domain.
ManyBooks.net - The best ebooks at the best price: free! Browse through the most popular titles, recommendations, or recent reviews from our visitors. Perhaps you'll find something interesting in the special collections. There are 25,689 eBooks available here and they're all free!
Project Gutenberg - Project Gutenberg is the place where you can download over 30,000 free ebooks to read on your PC, iPhone, Kindle, Sony Reader or other portable device.
World Digital Library - The World Digital Library (WDL) makes available on the Internet, free of charge and in multilingual format, significant primary materials from countries and cultures around the world.