Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
Project Gutenberg - Project Gutenberg offers free ebooks to download.
calibre - Ebook manager, viewer & converter
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.
Okular - Okular is a universal document viewer based developed by KDE.