A seller on ebay is listing a ton of rare promotional items and samples. The one that caught my eye was a promo cartridge for Earthbound NES. That's right, NES.
Nintendo made a few of these Earthbound NES cartridges to show a shows like E3. Earthbound NES was never released because Nintendo decided to focus on the Super Nintendo. Most of these promo cartridges were destroyed but this one lives on.
It is definitely rare and a great collectors item. The ebay seller says the last one sold for $1,000. I can't verify that, but that price sounds about right for an item like this.