Google Play Books - Google Play Books lets you search, preview, and buy millions of books using Google Book Search.
Rocket Reads - A simpler alternative to Goodreads
Amazon Kindle - Amazon Kindle software lets you read ebooks on your Kindle, iPhone, iPad, PC, Mac, BlackBerry, and...
Knowall - Book Recommendations from Friends
LibriVox - free public domain audiobooks
Kooba - An interactive graph for finding new books