LibraryThing is my obsession of the day.
It's amazon.com done right the community way. Think of it like flickr but only books. You can search and add the books which you have read and found interesting and tag them. This way you make a book-list of yourself. You can see mine here.
Once you do that you can discuss with the community which liked that book and see what other people have to say about it. Members can also write mini-reviews and rate the book.
LibraryThing has exploded in the online scene and they have already surpassed Amazon in the number of tags. Also, like in flickr they also have communities for books where members can discuss books which gives it a book-club like feel. Here's the Green Dragon community for Tolkien, it's the largest and the most active community. Go Join!