Delicious Library is a non-free Mac application that catalogues your library of books, movies, CDs, etc. It does this in an inspired way: apparently you simply hold up the item to your webcam (or iSight as the Mac people call it) and it will read the barcode and discover what the item is automatically.
Clearly, this functionality is too useful to remain Mac-only: what we need is an open source library that can recognise barcodes from webcam images, and then an open UPC database (similar to CDDB of old). This would be very useful for cataloguing ones library of – well, anything, really – and it would also open up social networking possibilities for people who want to publish their library catalogues.
Why doesn’t this open source software exist already (or does it)? There seems to be upcdatabase.com out there, and a few barcode recognition libraries (many of which are meant for your pocket PC or mobile phone, presumably so you can scan barcodes at the bookstore and get an instant online price comparison).