This is a monophonic single-oscillator synthesizer I cooked up as an example of driving Web Audio from Web MIDI.
Check it out, feel free to fork, submit pull requests, etc.
As of June 2014, the only browser supporting this is Chrome, and you must turn on the #enable-web-midi flag in chrome://flags. It should then work across OSX, Windows, Linux and Android, provided you have appropriate hardware.
Hosted at http://webaudiodemos.appspot.com/monosynth/.