If you are an avid reader like me (I'm bookaholic) certainly you'll enjoy these places.
The Shelfari is the best place to show off my books, the favorites, the owned, the wish, the must read ones, etc. There are interesting groups and you can connect with people all around the world just like you. The shelfs can be displayed in your blogs, journals or sites.And it's all for free.
The Library Thing is a very good site too, but for free you can catalog only 200 books. In the order hand, Library Thing have a great BookSuggest searching that can recommend you a book from a book...