Goodreads

When recommending titles, I believe that Goodreads looks at all the books you've read and dumps them into categories.  I read that if you don't rate the book or don't give it a high enough rating, then Goodreads won't recommend books for that category.  Then I think it goes by key words in the title and descriptions.
I liked that in both the cover or list view you could see what books you had read that had been the basis of their recommendations.