LibraryThing allows you to catalogue your own book collection online, adding tags, reviews and ratings that the entire LibraryThing community can share.
As with several of the Things, I can see why some people want to use LibraryThing but it's not for me. Although I read a lot I am fairly ruthless at getting rid of books unless I love them so I have a small, managable collection that does not really need cataloguing. I know what I've got and where it is, supplemented with transient additions from charity shops and the library that only stay in my collection for a short while.
I did sign up to LibraryThing, mostly in the hope I could use my mobile phone as a scanner to import books and that it would make a cool noise as I scanned book barcodes. Alas although you can (I think) scan using a phone you need to install stuff* to make it work so I gave up on that plan.
I was disappointed with the 'add books' function; I wanted to add 20 books in the same series (Beast Quest if you're interested) and I thought I would be able to mark a list then add them all at the same time but instead I had to do each one individually. Although this is probably a plus for anyone who is interested in genuinely using LibraryThing because as you add each title you get the chance to edit the bibliographic information, add tags, add a review etc.
I was pleased to see that it automatically grouped my books into series, and also showed which other books in the series I was missing. The 'if you like this you might also like...' suggestions were interesting and came up with a couple of authors I hadn't heard of.
*I've forgotten what exactly. I was tempted to put 'apps' instead of 'stuff' so I sounded like I knew what I was talking about but decided to go with honesty in case it's software not technically classified as an app.