calibre - Ebook manager, viewer & converter
Sumatra PDF - Sumatra PDF is a slim PDF/DjVu/EPUB/XPS/CHM/CBR/CBZ/MOBI viewer for Windows.
FBReader - FBReader is an e-book reader for various platforms. Features:
MuPDF - MuPDF is a lightweight PDF and XPS viewer.
Amazon Kindle - Amazon Kindle software lets you read ebooks on your Kindle, iPhone, iPad, PC, Mac, BlackBerry, and...
Foxit Reader - Foxit Reader is a free and light-weight multi-platform PDF document viewer.