What was the very first product to display a bar code on its' packaging?

gum... sort of.... it was the first item to be purchased with a barcode reader, but the rest of the supermarket was filled with other items with bar codes. There were also earlier 'bar codes' that used a bull's eye-like pattern instead of lines, but these were not standardized across supermarkets.

Wrigley's gum was the first product know to be sold using the bar code system we know today. What is not known is the size of the package.

I just love when I answer, then search. Allow me to correct myself. (Before others do it.)

In June 1974, one of the first UPC scanner, made by NCR Corp. (which was then called National Cash Register Co), was installed at Marsh's supermarket in Troy, Ohio. On June 26, 1974, the first product with a bar code was scanned at a check-out counter. It was a 10-pack of Wrigley's Juicy Fruit chewing gum. The pack of gum wasn't specially designated to be the first scanned product. It just happened to be the first item lifted from the cart by a shopper whose name is long since lost to history. Today, the pack of gum is on display at the Smithsonian Institution's National Museum of American History.

The site also confirms Methos' statement about the Bull's Eye coding system.

Dorian, you are correct...Wrigleys to be specific