Here’s a great site I came across this afternoon – Shelfari – allows you to create your own “virtual bookshelf”of books you are reading or have read, linking you with others who have read the same book, and creating a virtual book club in the process!
I’ve created a simple shelf with some of the titles I refer to regularly in the work I do – link to it here!

As an extra note, Stephen Harlow pointed me to LibraryTwoPointZero where the writer compares three Social Cataloging Applications: Gurulib, Librarything, and Shelfari.