Audible - Audible is the world’s largest provider of premium audiobooks and spoken word content.
calibre - Ebook manager, viewer & converter
LibriVox - free public domain audiobooks
FBReader - FBReader is an e-book reader for various platforms. Features:
DigitalBook.io - Free audio books and eBooks. Search, browse, listen, read and download over 100,000 free audio books and eBooks.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.