Since it was in the news as it got purchased by Amazon, I just joined Shelfari and added some books that were sitting on my bookshelf. I'm not sure how useful this will be, but feel free to friend me and we can swap book recommendations. Not that I read much nowadays, but I try to get a book in once every few months.
Some people collect books for some reason in personal libraries, and then lug them around from place to place forever, but I think that's insane. I'm a book "giver", usually trying to get rid of books as soon as I read them. A nice virtual bookshelf like this will do me perfectly, actually. (And a library card for my local library, of course, in case I want to check the books out again.)