[HoopyFrood]

I actually use a different one, Shelfari. I can't say how it compares to the other sites, because I haven't been on either of them, but on Shelfari you can build your 'bookshelf', tag the books under whatever headings you wish, make friend requests and see which books you both have, add opinions and reviews to books, create discussions, join groups. I feel like I hardly take advantage of what the site has to offer, but you could just spend hours going through all the books that you own, your friends' books, adding comments everywhere and etc...