At the last western Mass Drupal Users Group meeting, we were given a demo of UberPOS.  It hooks into UberCart, which is an eCommerce element of Drupal,  UberPOS provides a POS system which includes connectivity with hardware such as a barcode reader and cashdrawer. 

The demo we were given showed how an SKU was read into the UberCart inventory system via the barcode reader.  Then, a purchase was made, again using the bar code reader to get the SKU from the product.  A receipt was generated and printed.  When the receipt finsihed printing, the cash drawer opended. 

For more on this system, check out the UberPOS site.  It never ceases to amaze me what people are doing with Drupal.