LibraryThing, whose Unsuggester Geoff Nunberg mentioned, is an interesting idea. I learned about it from Steve of Language Hat. Claire of Anggarrgoon also uses it.
I've been using it to catalog my own books. It is handy since you can enter just the ISBN or title or author and it will find anything that matches in one of a large selection of sources. It is intended in part to make book-owning a social activity. You can find out what other people have (there's an option for making your catalogue private if you wish), and you can find out how many other people own any particular title. I know that my tastes are somewhat idiosyncratic, but I was still a bit surprised at how few people have some of my favorites.
There are currently 116,319 members with a total of 7,995,319 books comprising 1,486,839 distinct works. I can understand that not a lot of people will own モンゴル語四週間 (Mongolian in Four Weeks, in Japanese), but I am disappointed that I am the only owner of Morris Halle's The Sound Pattern of Russian and Ferdinand de Saussure's Mémoire sur le systeme primitif des voyelles dans les langues indo-européennes. Shouldn't more people have Regna Darnell's Edward Sapir: Linguist, Anthropologist, Humanist? Not one other person has Josep Nadal and Modest Prats' Història de la llengua catalana! And isn't it shocking that only one other person owns Brent Berlin's Ethnobiological Classification? Only eleven other people have a copy of the Hōjōki (方丈記), whose opening paragraph is, in my opinion, quite possibly the most beautiful passage ever written. (I have two editions, plus an English translation.)
7,245 people have The Da Vinci Code. Go figure.