I mentioned it before today: I started to create my virtual book shelf for technical books in LibraryThing a few days ago. It allows you adding new books by simply selecting them from a list. By entering the title, author or ISBN number you just query one of many book directories such as the Library of Congress or simply amazon.com. If you don't find your book (e.g. in my case a couple of German books) you can add alternative Amazon websites. There are some interesting features in this kind of features I am interested in: (1) you can export your information to tab-delimited or CSV files. (2) You can easy access all the book information including cover images without typing all the information in first. (3) LibraryThing provides an API to access the information. I haven't found time to check the API yet, but it allows you  to receive simple information based on the submitted ISBN, title or language of the book.