Believe me I have seen a lot of cool apps over the years but for personal use I reckon Delicious Library is the best of the lot. I place one of my books with it's barcode in front of my iSight camera and Delicious Library scuttles off and collects all the metadata from Amazon and builds me a virtual library. It integrates with Address book and iCal and simply by dragging a book onto someone's name card it records it as loaned and shows when it should be due back! Works for VHS's DVDs and CD's.
I have hundreds of books at home and this has made me so, so happy!
Scanned in 161 books - now all sorted in shelves (virtually) - one bookshelf down five more to go!