https://musiclab.chromeexperiments.com/Song-Maker/

CHROME MUSIC LAB: SONG MAKER

To make a song, add notes by clicking the grid. Then, share your song with a link. You can also use a MIDI keyboard or sing a note into your mic.

You can also use your computer keys. Press the arrow keys to move, enter and backspace to add and remove notes, and spacebar to play.