This is something we have been planning for a while but finally got round to implementing it: UPC barcodes for each product on UPCZilla. You can see an example here:
The barcodes are generated using the Picqer PHP Barcode Generator library. Currently they are generated on-the-fly as caching them is not an option (our server would collapse under the weight of hundreds of thousands/millions of image files) but if there seems to be too much overhead in generating the images every time then we will try to think of something else.
In any case, we hope someone will find this addition to our UPC/product information useful. You can read more about UPC-A barcodes here: http://www.barcodeisland.com/upca.phtml