This is a really cool website for all things book related. You can search any title or author and find out everything you wanted to know about any book you might be interested in. Each book has its own information page with reviews, synopses, and a list of members who also have an interest in this book. You can become a member and create your own page with a picture and information about yourself and your reading interests. You can add 'books' to your 'bookshelf', a wonderfully realistic visual gadget, with your own information, ratings, and notes. You can request friendships with other members, form or join a group around a certain genre or interest, post or join discussion threads; and peruse others' bookshelves, which will even list books you have in common.
It's such a great way to keep a database of all the books you've read and all the one's you'd like to read. I don't touch a book now that I haven't first researched on shelfari. There is no commerce on this site; it's only a place to collect information, probably a good thing given my weakness in bookstores.
I know this all sounds like a sponsored plug, but it isn't. It's just me indulging my hobby.
There are a couple of bookswap sites that look interesting too, like BookMooch and PaperBackSwap. They kind of look like shelfari but with the option of swapping books through the mail - a great way to keep yourself in books if you don't have access to bookstores and libraries.