I'm a librarian who loves anime, manga, and reading a wide variety of genres.
One of the book sites I use is LibraryThing, which, once you get past the learning curve, has some nice features. I just found out that the "Vous et nul autre" feature (found under: Home > Stats/Memes > Memes) got an update a couple days ago. It used to just show you which books you shared with one other member. Now it shows you which you share with 0, 1, 2, 3, 4, or 5 other members.
Maybe useless to some, but I like it as a "What should I read and review next?" tool, since sometimes it's nice to review something that dozens or hundreds of people haven't already added to their collections and reviewed.
(Cross-posted from my blog)