If you like this book, you might hate this one...

Courtesy Gazebo, I just found out about LibraryThing's UnSuggester. If you enter a book you have or have read, then (provided at least 75 people have listed it), you'll be given a list of books that you definitely shouldn't read because you'll probably hate them.

For example, if you like the Feynman Lectures on Physics (approximate title), then you shouldn't be reading anything by Stephen King.

Or if you like Oxford's new annotated bible, there are a number of fantasy/sci-fi authors you should avoid.