I've seen LibraryThing before and actually thought I'd created an account, but I guess I was mistaken. I do have an account with GoodReads. It's pretty amusing to me that they're selling the CueCat that you used to be able to get for free from RadioShack. The price is decent though, compared to my school barcode readers--I wonder if it's compatible with Destiny! I love that it will display a book cover and is integrated with Amazon and Library of Congress.
One thing I like about GoodReads (and LibraryThing may have the same functionality, I just didn't notice it) is that if you're willing to give up your book to someone who wants to read that, you can indicated it on your list. If you don't covet books and you're willing to give up what you've got, GoodReads might be better for you. I think it's a cross between LibraryThing and BookCrossing.