calibre - Ebook manager, viewer & converter
Documents by Readdle - A central hub for all your files.
FBReader - FBReader is an e-book reader for various platforms. Features:
Documents Pro 7 - The ultimate all-in-one mobile office suite for the iPhone and iPad with 8 premium apps in 1.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
GoodReader - With GoodReader on your iPhone, iPod Touch or iPad, you can read virtually anything, anywhere.