Via Diane Greco: it's LibraryThing: social software for your bookshelf. del.icio.us-like tagging; Flickr-like blog widgets; Amazon-like book reviews; commenting on others' libraries; discussion fora; author pages; RSS feeds; Z39.50 library searching; &c.; &c. This is heroin for book junkies. Of course I signed up right away and have been furiously cataloging my library ever since.