The Rational Keyboard is a browser app to play with harmony on the rational number line. Imagine a piano with infinitely many keys, one for each rational number, and the keys move around and resize based on what sounds good, using some really cool math.
Audio is synthesized in background Web Workers and played with HTML5 Audio objects, and the interactive visualizations use a 2D HTML5 Canvas. Check out the source at github, or just grab the audio encoder for your own apps.
Audio seems to work best in recent Firefox and Chrome releases, but HTML5 audio is still a young technology. Sometimes pausing helps to unglitch audio.