A few years ago I saw the great Tone Matrix tool by Andre Michelle. It's a flash based audio experiment that let's you highlight items in a matrix to create music.
With newer browsers, you are finally able to implement some of the things you could do in flash years ago, without requiring browser plugins. I created Audio Grid as an experiment to explore this fact.
It's using timbre.js for it's core functionality, but is also making use of a few libraries that I recently created for use in Audio Sort:
soundfont2mp3 - a command line utility that lets you extract single note mp3s from soundfont files.
timbre.soundfont.js - play soundfont urls using timbre.js
timbre.mp3_decode.js - a timbre.js plugin that decodes mp3s using jsmad
free-midi - a collection of mp3s generated by soundfont2mp3 and the S. Christian Collins GeneralUser GS Soundfont.
You can see a demo of Audio Grid by clicking on the image below: