Online retailer Amazon is now offering digital downloads of "over 50 Nintendo digital game titles" which can be downloaded directly to your 3DS or Wii U console using a "seamless redemption experience".
It works like this. You purchase the game on Amazon as you would normally, and then link your Nintendo Network ID. A code will be created and by clicking the "redeem" button, you can send it straight to your system.
The selection of games available now includes retail releases, Virtual Console games and eShop titles. You can browse the full library here.
The service is limited to North America at present.