The Unsuggester at LibraryThing.
Unsuggester takes "people who like this also like that" and turns it on its head. It analyzes the seven million books LibraryThing members have recorded as owned or read, and comes back with books least likely to share a library with the book you suggest.
If you own or have read Dee Brown's
Bury My Heart at Wounded Knee you will not own or have read the
Harry Potter Boxed Set. If you own or have read Clive Barker's
Books of Blood you will not own or have read
The Divine Secrets of the Ya-Ya Sisterhood. If you own or have read
The Selfish Gene you will not own or have read Virginia Andrews'
Flowers in the Attic. Well, thank Christ for that.
Via The Language Log.