I, er, already catalogue my books, largely because I want to be able to see them as an ordered whole and, until I moved to a new flat with Margot last week, they were stacked vertically in no order in a cupboard. I have a master catalogue, ordered alphabetically by author, and a few subject catalogues, following the Chicago Manual of Style bibliography rules. They're just Word documents for now, although I intend to convert them into XML.
I like the idea of sharing my collection with other people in LibraryThing, but not enough to add all my books (about 500 volumes) to my account. Instead I've started to transfer my biggest subject catalogue (150 volumes). It's quite good fun, although searches in a collection seem slow and unreliable, and the data is terrible.