calibre - Ebook manager, viewer & converter
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
FBReader - FBReader is an e-book reader for various platforms. Features:
Apple Books - The all-new Apple Books has been redesigned to make finding, reading, and listening to books a beautiful, effortless experience on iPhone and iPad.
Project Gutenberg - Project Gutenberg offers free ebooks to download.
Cool Reader - Fast and small cross-platform eBook reader for desktops and handheld devices