calibre - Ebook manager, viewer & converter
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
FBReader - FBReader is an e-book reader for various platforms. Features:
Zotero - Zotero is a free, easy-to-use tool to help you collect, organize, cite, and share research.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
Qiqqa - Qiqqa is a free research and reference management software. It can be used in many organizational projects from the academic to the personal to the business endeavor. Read more about Qiqqa.