Google Books has a new feature: "My Library". It's a very basic system that allows you to keep a list of the books you own (provided they are listed in Google Book Search), together with labels, ratings, and reviews.
I really like the idea; I've been trying out various 'social library' sites in an attempt to find one that I could use. It's not as easy as it might seem, since many of them either require some form of payment or lack features I need. Of course, google books library also lacks features I need, but I can at least be fairly certain it won't disappear when the creator goes broke.
I'd be willing to forgive the lack of features, since it's pretty likely that google will be adding more features as time goes by, but there is one difficulty: not every book I have is listed in their index, and there's no way for me to add a book myself. Now, it's totally reasonable that they don't just let anyone add a book--the database would be quite useless fairly quickly. However, it's missing some books that I would really expect to find; for example, the hardcover editions of the Harry Potter books that I bought a few years ago aren't in there (nor, I think were the paperbacks). There is almost certainly some other edition of those books listed, but I want to add the correct edition.
Still, I think it'll be worth giving it a try. Maybe if google pushes this, it'll be successful and useful. I hope so, anyway.