I've used LibraryThing since my senior year of high school and have always loved it. As others have noted, unlike Goodreads, LibraryThing isn't a social network. I'm an obsessive about what books I own, how to categorize them, where/when I bought them, where/when I read them. I don't care for my friends to know what I've been up to and I don't need their recommendations (at least not through the web).

As an aside, I vaguely recall that at some point their memberships were pay-what-you-can. Super curious what the distribution of payments looked like.