I love Library Thing. You wouldn't know it from my track record (one lost account and another inactive one until 2.0 told me to do it) but it really is a great idea. I've told my parents about it, suggesting that this be a way to finally categorize their personal library, but they haven't quite caught on yet (plus I think they might have to upgrade to a paid account). The only thing I don't like about Library Thing is that I always want to update it when I am nowhere near my books and cannot remember what I own.
That reminds me, to clarify for myself and others, here is what I use the different book-tracking devices for.
1) Library Thing: books I own.
2) All Consuming: books I'm reading right now.
3) Facebook: books I'm reading right now, want to read next, and have at some point read.
(Really, I could eliminate All Consuming on this task and let Facebook do all the work, but I can put All Consuming on my blog and if I can do the same with Facebook, I haven't figured it out yet.)
Another thing about Library Thing I like - I can put random books from my library on my blog (see left) and I can customize this widget to include the size and number of books I want shown at one time. This widget also links back to my account, so you can all check out my library (it's not comprehensive by any means, but it's a good example). I think the site itself is easy to use, and it's a ton of fun to see the covers of the books spread out in front of you.
I could see a site like this with movies instead of books becoming popular (there's probably one already out and I just don't know about it yet, isn't there?). I suppose All Consuming could be considered something like that, but really, I don't like All Consuming as much, and I like how Library Thing is just books, so there should be a just-movies site, too. I'm rambling, I know - forgive me, I haven't been feeling well. Next up is Twitter!