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I use Library Thing. I was signed up for a while before I actually used it. It's a bit daunting to catalog a library. Once I started, though, I was hooked. It's so easy when you type in the ISBN number. I also like seeing what other friends have on their shelves and who shares my tastes. I'm signed up as Chrisbookarama, if you'd like to add me as a friend.
I've checked out Shelfari. It didn't really appeal to me. I heard they had problems with spam lately as well. I've signed up for Good Reads (haven't used it yet) and Chapters (the book store). I'm not impressed with Chapters. You can't browse their groups. I don't get that. Plus, it's difficult to navigate. I think they need to work on it.
Really, there's a catalog for every book lover. Kimbooktu tested several. Check it out here.