I saw this mentioned on another site, and the general consensus was that most people forgot about this, but does anyone who bought the Wii U deluxe set remember the "Deluxe Digital Promotion"? Basically, if you bought the deluxe set, you had the option to get points for your digital purchases. For every 500 points, you get a $5.00 credit in the eShop. So, everyone knows about the free game you can get when you register your copy of Mario Kart 8. What most people don't know is that whichever free game you choose to download will give you points in the Deluxe Digital Promotion. As an example, I chose The Legend of Zelda: The Windwaker as my free game. The game costs $59.99 in the eShop if you were to purchase it, so the game gives you 599 points in the promotion, which is enough to get any NES game download. So basically, if you buy Mario Kart 8, you can actually get two free games. Go here to check your balance, you may have a free game waiting.
https://p.nintendo.net/deluxe