A pretty buggy app that's intended to automatically harmonize with your singing. For now, you can play with it by cloning the repo and serving it on localhost. This uses pitch.js and fft.js.
git clone --recursive https://github.com/echentw/autoharmonizer.git
cd autoharmonizer/
python -m SimpleHTTPServer 8080
and you can access the app in your browser at localhost:8080.