calibre - Ebook manager, viewer & converter
LibraryThing - A home for your books.
FBReader - FBReader is an e-book reader for various platforms. Features:
Readernaut - We are a small community of readers, sharing our adventures and supporting each other as we persue a life of learning.
Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
Riffle - Just good books and the readers who love them.