Though a 3DS eShop version is on the way, DSiWare app Petit Computer remains as one of the most intriguing downloads available on the last-gen portable. It allows you to create games using BASIC code that are fully playable on the DSi, and they can then be shared through QR codes.
It's seen its share of terrific projects, but a recently released fan-made effort from Lazer Light Studios is possibly one of the very best yet. "Super Fighting Robot!" is a remake of the iconic Mega Man 2, which some argue is the very best of the 'Classic' NES entries. Version one is available now and requires the scanning of a whopping 243 QR codes, but importantly it actually works; it has a main game and also a built-in level editor. It's been a long time in the works, and though early-adopters are encountering some bugs — in particular infinite lives when jumping from the editor to the main game — it seems like a remarkable project.
If you have Petit Computer and the willingness to give this a try, it could be a fun moment to play a new version of Mega Man 2 on the DSi.