Goodreads. With Goodreads, I can share, rate, organize and keep track of what books I am reading. (I love lists and I love organizing, which are probably a big part of why I love this site.) I can also see what my friends are reading and what they thought of books that we both have read. I can write reviews for books that I have read and post them, as well as read reviews for almost any book under the sun. After reading a book I can send a recommendation to any of my friend who I think would also enjoy it. Before I discovered Goodreads, I had dozens of scraps of paper about with books written on them that I wanted to read, now I just put them on my to-read list. (Or sometimes my friends will just send me an on-line recommendation.) It cuts down on the clutter. In addition to these, Goodreads makes it easy to form your own online book club. If you love to read and you haven't checked this site out yet, you should.
Another fun site for you bibliophiles out there is Book Drum. It's almost like a collection of elaborate creative book reports. People take books they love and create profiles to accompany them. A profile is full of comments, pictures, videos and music that are relevant to the book. The only drawback of this site that I can easily get lost in and get distracted from those other productive things that I should be doing.