calibre - Ebook manager, viewer & converter
Open Culture - Discover thousands of free online courses, audio books, movies, textbooks, eBooks, language lessons, and more.
FBReader - FBReader is an e-book reader for various platforms. Features:
Project Gutenberg - Project Gutenberg offers free ebooks to download.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
LibriVox - free public domain audiobooks