Archive.org - Internet Archive is a non-profit digital library offering free universal access to books, movies...
ManyBooks.net - Thousands of free ebooks, pre-formatted for reading on your computer, smartphone, iPod, or e-reading device - ePUB, Kindle, eReader, PDF, Plucker, iSilo, Doc, RTF, Mobipocket, Newton Paperback, and zTXT ebooks ready to go!
Open Culture - Discover thousands of free online courses, audio books, movies, textbooks, eBooks, language lessons, and more.
Foundcy - What are founders reading today?
Project Gutenberg - Project Gutenberg offers free ebooks to download.
calibre - Ebook manager, viewer & converter