I heart librarything
I'm not a cataloging freak, and organizing my own books is not even close to making it on one of my to-do lists in the next 10 years, but I love this as a way to let others know what you're reading, what you've loved reading, and/or what books are in your collection (for those who love to buy me books, but forget what I have...this just happened last week when my Dad was in Harvard Square and was going to pick up the new Melville biography for me, but didn't know if I had already bought it...). Plus, I want to get my nieces' library in here so I know what books they have already acquired. They live in NY, and I'm always wanting to buy/send them books, but I often avoid buying some of my favorites because I think they might already own them.
Plus, they've added so many tools...nice way to add your own tags, comments, and blog widget (mine's on the right side of this page). Love it!