It's called Shelfari (here's the site URL: http://www.shelfari.com/ ) It's free to sign up and allows users to create their own bookshelf.. great for keeping track of books you've read, or adding ones you plan to read in the future. You can then rate, review, and add tags to books on your shelf for easy sorting. There's also a bunch of groups centered around different genres, authors, etc so you can get reading suggestions and reviews from other members. In my opinion, it's a really useful site; I suggest you all check it out.
You can see my profile/bookshelf here: