From unFun Games:
Mario Paint was one of the greatest classic games of all time. The music writing system was the real standout of the package...but it was severely limited.
So here's Mario Paint Composer! All the fun of the original Mario Paint music system but now with expanded features like: 5 notes per beat, 4 times the song lengths, sharps and flats (hold down shift/control to change them), volumes for each beat, and the ability to save an unlimited amount of songs.
You can also take your masterpiece online and share it with other users. Listen to other songs and create your own list of favorites so you can listen to them anytime you want.
This game is free, and is not affiliated with Nintendo in anyway.