CoilSnake is the most powerful mod making tool for the game EarthBound. CoilSnake has been used to create several original games in the EarthBound engine, as well as to translate EarthBound:
Latest Version: 4.0
On Windows, just download the .exe. Mac and Linux users may install from source by following the instructions in DEVELOPMENT.md.
The CoilSnake tutorial is the best place to get started.