I've been playing with LibraryThing a bit. It's a web app for cataloging your books. It seems pretty good.
I had written a small library application with Suneido but it was a lot of working entering books. So I decided I should be scanning the barcodes and looking them up. Buying a scanner was tough because there are too many kinds and I didn't know what would be best. I ended up buying an AS8155 from Custom Sensors Inc. It's a CCD scanner with a USB interface. I have no idea if this was the right choice but so far it's been working well.
I started to add barcode lookup to my Suneido application, using isbndb.com and got it more or less working. But then I thought it would be nice to have cover images from Amazon. So I started looking at Amazon's web services.
Then I realized that, interesting as it might be, this wasn't actually getting my books cataloged! So I thought I should look for an existing application, which led me to LibraryThing. There are quite a few choices in this area but it looked reasonable.
You can sign up for free and enter up to 200 books. I set up an account for work and scanned a bunch of books that happened to be handy (some recent, some not). You can see the results at: http://www.librarything.com/catalog.php?view=apmckinlay (at least I think you'll be able to see them since I made it public).
The barcode lookup works 95% of the time. I had a few older books with no barcode, a few books I bought in Asia it didn't find, and a few I had to use amazon.co.uk to find. And a few novels at home where the UPC code on the barcode instead of the ISBN (confused me for a bit since they print the ISBN number as well).
But even when the barcode lookup fails it's pretty good at finding books from title and/or author, even partial.
So far the only complaint is that I can't figure out how to search for C++. It keeps wanting to strip out the "++", even if I enter it in quotes (it seems to take the quotes literally).
I like the ability to "tag" the books (although I haven't used it yet). From del.icio.us and gmail I've gotten to like tagging stuff.
LibraryThing has other features that I may use - like recommendations - and some that I probably won't - like social stuff. But so far it looks good. If you're looking for this kind of thing, I'd recommend it.
For a discussion of alternatives see this discussion from Joel Spolsky's blog