gbawriter is a writing and diary tool for the Game Boy Advance. 
Documents are saved as .txt files on the SD card.

It uses a custom controller-based typing system that makes it possible to write using only the GBA's D-pad and buttons. Instead of an on-screen QWERTY keyboard, letters are arranged into small directional groups that are designed to become quick to use once the controls enter muscle memory.

Code:
https://github.com/Xinon232/gbawriter