A bunch of my friends have been using LibraryThing to create an online catalog of their books, share that catalog with others, and so on.
I can see the appeal, and at some point I may sign up for a paid account. I did sign up for a free account just to see what it was like, and I imported 200 of my books (which is all you can have in the free account). It took a long time to import them, though, and I wasn't thrilled with the results; the interface feels a little clunky to me.
Now it turns out that, as of a week ago, there's a similar (but comparatively minimalist) feature available for free as part of Google Book Search: the My Library feature. If you have a Google Account (which you do if you have a Gmail or Orkut or Picasa Web Albums account), then every time a book that you own comes up in a book search on Google Book Search, you can click the "Add to my library" link to put the book into your own personal catalog.
When you're looking at your own library, you can choose to view it in "list view" or "cover view." You can also import books, by ISBN--I just pasted in a list of 800 ISBNs, and it added the books to my library in seconds.
Unfortunately, there are flaws, as you'd expect from such a new feature. It couldn't import nearly a third of my books, presumably because it didn't recognize the ISBNs. (To be fair, probably some of the ISBNs are inaccurate, or are old-fashioned SBNs.) There's no sort feature; as usual with Google stuff, the expectation is that you'll do a search for what you want to see, rather than browsing through in any particular order. (The search includes the content of the book as well as the metadata; that may or may not be what you want.) Some of the cover images are broken, and many are missing. Some of the metadata is incorrect. Viewing only ten books per page in cover view looks weird. I'll be filing a bunch of bugs.
And it certainly doesn't yet have many of LibraryThing's features, especially the social features having to do with comparing your library to other people's. You can provide others with a link to your library (here's my library), but the URL contains a long and awkward random-looking strings of letters and numbers.
(Note that that rendition of my library probably contains quite a few books of my father's that I'm giving away. At some point, when I get my act together, I'll post--either at GBS or at LibraryThing, probably--the full list of my father's books that I'm giving away, and y'all can let me know if you want any of them. But it's been months since I started planning to do that, so don't hold your breath.)
And I could be wrong, but I don't think there's a way to attach a note to my copy of a book, as opposed to Google's metadata about the book in general. In other words, if I want to say "I loaned this book out to so-and-so" or "This book is on a shelf in my bedroom" or "This is one of my father's books that I'm giving away," there doesn't seem to be a way to do that.
But the feature's only been available for a week, and I do quite like it so far. I look forward to seeing what happens to it as time goes on.
What I really want in an online catalog is tight integration with Delicious Library. Well, that and much more accurate sources of metadata. DL gets most of its metadata from Amazon, which is wildly inaccurate much of the time. Also, I'd like a way to easily input old (pre-ISBN) books. And more information about the contents of short-story collections and anthologies. (Google Book Search seems to generally be better about this than, say, Amazon is, from what I've seen so far.)
Also, I'd like a pony.
. . . Wow--Google Book Search has a whole bunch of cool features I didn't know about. You can write a review of a book, give it a rating from 1 to 5 stars, and attach labels/tags to it. There's a "find this book in a library" link (which takes you to WorldCat, which I had never heard of before); there's a "Places mentioned in this book" map (something I almost proposed creating by hand recently); there's key terms that appear in the book, and related books, and popular passages, and a bunch of other stuff. It's worth taking a look--go to Book Search and search for a book or author, then follow the links and look around.