Favorite new site: aNobii — it's to bookworms what Facebook is to socialites! I've tried out virtual bookshelves before, with Google Books' "My Library" option being quite good. But where Google's focus is to make it possible to browse and search inside the text, aNobii immerses you in a social network of readers.
Not long after adding your first few books (done by entering their ISBN numbers in batch), aNobii's "matchmaking" system will introduce you to other books you may enjoy — and to other people with likeminded tastes. It's lovely to browse through other people's bookshelves, and inspect books that you might enjoy yourself. It's almost like a huge virtual bookclub.
The social linking is done very nicely: if you strongly identify with a complete stranger's taste in books, you can tag that person as a "neighbour" (someone living close to you in L-space, but whom you don't know in real life). You can then be notify when they update their bookshelves, for example by finishing new books and ranking them on a four-star system.
If you enjoy reading, and enjoy being exposed to new books by word of mouth, you're likely to enjoy this excellent online resource.