Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
calibre - Ebook manager, viewer & converter
MuPDF - MuPDF is a lightweight PDF and XPS viewer.
FBReader - FBReader is an e-book reader for various platforms. Features:
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.