A couple months ago, I happened across a cool new site. It's called LibraryThing. Basically, it's a way to catalog all your books online as well as see other people's libraries. One of the neat features is after you put a couple books in, it starts showing you other people's libraries and the number of books you have in common. There are some groups and a bit of a social networking aspect of the site that I haven't fully explored yet.
There's also a unique feature called "UnSuggester". Unsuggester takes "people who like this also like that" and turns it on its head. It analyzes the books members have recorded as owned or read, and comes back with books least likely to share a library with the book you suggest. Some of the results are pretty accurate, but some are a little funny. For example, I entered the name of Ronald Reagan's autobiography. It returned Stupid White Men by, Michael Moore. Makes sense. But some of the other results are books from the Harry Potter series, Necromancer, Bridget Jones, and Memoirs of a Geisha.
Check it out.