LibraryThing is a hoot. I've actually been a member since early 2007, but it's been ages since I visited. I will need to make it a regular stop once again.
LibraryThing is, to start, an online cataloging tool. Amazon or Library of Congress can be searched and titles loaded into your online shelf. But what makes it so fun is all the discussion groups, of which "Librarians who LibraryThing" is the largest. There are all the standard groups you would expect--YA, sci-fi and so on, but then some real niche ones: BBC Radio 3 listeners, tea drinkers, self-described heathens. Then you can always feel bad for the communities with one member, like the "emo/goths can read" group that the creator describes as "We can prove to people thet [sic] we are not as dumb as people think even non emo/goths can join to help protest" Note to emo/goth can read group founder: my friend, you are on your own. Oh, but you knew that.
Apparently some libraries use LibraryThing as their catalog. This makes sense to me as a cost-effective way to get it done, and that it may be particularly well suited to the small specialized collection.
I must confess I somehow got sucked into Good Reads at the expense of LibraryThing. Good Reads is kinda-sorta the same thing. I feel a little shameful about it, now. I feel like the good-looking jock in a John Hughes movie who knew the dorky girl and liked her fine but then pursues and becomes besotted with the cheerleader only to find at the conclusion of the film that the cheerleader has no soul and the dorky girl is actually what he wanted all along. Yes, that's exactly what I feel like.