I had seen LibraryThing before, but more in the context of LibraryThing for Libraries, for which I tried, but failed, to get a grant for our library. It was really interesting to go back and look at it through a reader's eyes. I had never had a reason to catalog the books I own, so didn't think I would use this, but I got an idea from the blog of someone else doing North Texas 23 Things--keep track of the books you've read and get suggestions for similar books through LibraryThings.
I think that's a great idea! So I've signed up, looked around, put in my bid for some "Early Review" books and some "Member Giveaways". It's such a great book community!
I joined the Librarians Who LibraryThing group and looked at some other groups, as well as the ThingLang, ISBN Check and MarcThing. I would probably be more interested in those applications if I were still a cataloger, but then I'm not really a cataloger at heart (i.e. appallingly un-detail-oriented). I've bookmarked LibraryThing and will definitely spend more time there.