calibre - Ebook manager, viewer & converter
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
Amazon Kindle - Amazon Kindle software lets you read ebooks on your Kindle, iPhone, iPad, PC, Mac, BlackBerry, and...
FBReader - FBReader is an e-book reader for various platforms. Features:
Project Gutenberg - Project Gutenberg offers free ebooks to download.
Scribd - Unlimited books, audiobooks & comics. Unparalleled discovery. Any device. $8.99/month