Book cataloging web site LibraryThing's new tool, called UnSuggester, recommends books to you based on the books you dislike.

For example, if you hated the book Eragon, the #1 suggestion is Please kill me : the uncensored oral history of punk. After running it through the paces, I'm not sure how useful or accurate it is, but it's an interesting idea.

Check it out here: http://www.librarything.com/unsuggester