Library Thing

If you own a fair number of books or are interested in books, this is a must see website. It's almost as good as going to the bookstore and browsing, except you don't end up walking out with a handful of books you "just happened to find". I like that there are reviews by other members to give you a better idea about what a particular book may really be about. It makes it easier to make a wish list of new books. The site's search engine is very useful since you can search by "tags" as well as, title,author, etc., making it easy to find related books to any subject. There are also discussion groups to participate in, which would be much easier to do online, than holding a group you have to attend in person.