I have been a user of LibraryThing for some time, entering my catalogue of books and then slowly starting to take advantage of the social functionalities of the concept. It has been very useful in sharing my books with family and friends as well as learning about other books and authors I should check out.
In addition to the variety of interesting things that the site offers, I have found the mobile access to your personal book collection to be very useful. I can browse to LibraryThing from my mobile phone and I have immediate access to an organized, web-based catalogue of my personal library. From anywhere in the world, I can refer to what books I own, and access metadata, reviews, and ratings. I have done this from the book store, from the library, and even from work.
Call me a book geek, but I think that is very cool.