Library Thing is one of those resources I'd heard about for a few years but never got around to actually looking at until it was assigned as part of SPL's 27 Things learning project. And now, not only am I hooked, I'm pretty sure my bookworm husband is as well.
Library Thing is a way for readers to post their favorite books and network with others who have similar interests. As a reader, this is a great service because I like to read from a lot of different genres and areas, and library thing list others who have similar tastes so you can visit their posted collections, read their comments, and expand your own "to be read" list.
Library Thing also hosts hundreds of groups focused on various aspects of books and reading. Of course, the first group I joined was Librarians Who Librarything which includes discussions on whether or not to ditch Dewey, collections and services to babies and toddlers, community-wide book events, etc. Library Thing also pulls bibliographic data, so in addition to adding tags, the records pull in the authorized subject headings.
I don't have too much in my collection yet, but feel free to check out what I have.